The lower Lesser Himalayan sequence

The lower Lesser Himalayan sequence:
A Paleoproterozoic arc on the northern margin of the Indian plate
Matthew J. Kohn1, Sudip K. Paul2, and Stacey L. Corrie1
Department of Geosciences, Boise State University, Boise, Idaho 83725, USA
Wadia Institute of Himalayan Geology, Dehra Dun, India
1
2
ABSTRACT
The lower Lesser Himalayan sequence
marks the northern extremity of the exposed
Indian plate, and is generally interpreted as a
passive margin. Five lines of evidence, however, collectively suggest a continental arc
setting: (1) igneous intrusions and volcanic
rocks occur at this stratigraphic level across
the length of the Himalaya, (2) ages of intrusive and metavolcanic (?) rocks cluster at
1780–1880 Ma but also indicate a long-lived
igneous process, (3) detrital zircon ages in
clastic rocks cluster at 1800–1900 Ma, with
a unimodal age distribution in some rocks,
(4) the mineralogy and chemistry of metasedimentary rocks differ from typical shales
and suggest a volcanogenic source, (5) traceelement chemistries of orthogneisses and
metabasalts are more consistent with either
an arc or a collisional setting. Intercalation
of volcanic rocks with clastic sediments and a
general absence of Proterozoic metamorphic
ages do not support a collisional origin. An
arc model further underscores the profound
unconformity separating lower-upper Lesser
Himalayan rocks, indicating that a Paleoproterozoic arc may have formed the stratigraphic base of the northern Indian margin.
This, in turn, may indicate disposition of the
Indian plate adjacent to North America in
the ca. 1800 Ma supercontinent Columbia.
Felsic orthogneisses (“Ulleri”) likely represent shallow intrusions, not Indian basement.
INTRODUCTION
Understanding the origins and predeformed
geometry of the northern exposed edge of the
Indian plate is crucial for unraveling the deformation history attending collision of India with
Asia, and hence for reconstructing India’s position in former supercontinents (e.g., see reE-mail: [email protected]
†
views of Gansser, 1964; Le Fort, 1975, 1996;
Yin, 2006). The Lesser Himalayan sequence
plays a central role in both endeavors. It is
directly involved in several major Himalayan
thrusts, most significantly the Main Central
and Munsiari (or Ramgarh) thrusts. Interpretation of the genesis of Lesser Himalayan rocks
also figures prominently in the placement of
India in the hypothesized, ca. 1800 Ma supercontinent Columbia. Columbia, in turn, is important for understanding the supercontinent
cycle: did supercontinents form in the Paleoproterozoic and Archean, and if so what influence did they play in surface processes (e.g.,
Reddy et al., 2009)?
Metasedimentary rocks are reported to constitute the lower portion (≥~4 km) of the Lesser
Himalayan sequence (total thickness ≥~8 km;
e.g., Stöcklin, 1980; Valdiya, 1980; Schelling,
1992; DeCelles et al., 2001; McQuarrie et al.,
2008), and are generally interpreted as the passive margin sedimentary cover to the Indian
craton (e.g., Brookfield, 1993; Upreti, 1999;
Myrow et al., 2003; Gehrels et al., 2006).
Yet, in contrast to the passive margin paradigm, igneous events are also recorded within
the lower Lesser Himalayan sequence. Wellconstrained radiometric ages are sparse, but
zircon U-Pb and Pb-Pb ages for intercalated
orthogneisses and metabasalts, and detrital zircon grains from the entire breadth of the Himalaya indicate a common Paleoproterozoic age
of 1800–1900 Ma (Fig. 1, Table 1). This age
span is not consistent with zircon ages from the
Indian shield (Parrish and Hodges, 1996; this
study), so a fresh interpretation is warranted.
In NW India, plume or rift magmatism is commonly invoked (e.g., Bhat et al., 1998; Ahmad
et al., 1999; Ahmad, 2008).
In this paper, we present and discuss several lines of evidence to argue that the Paleoproterozoic assemblage at the base of Lesser
Himalayan sequence represents a continental
arc, rather than a passive margin, a collisional
belt, or a plume- or rift-related environment.
We interpret these rocks to consist predominantly of reworked volcanogenic sediments
interspersed with intrusive, volcanic, and
volcaniclastic rocks. Supportive data include
new field observations in NW India and central Nepal, previously published field descriptions across the Himalaya, new and previously
published chronologic results (Table 1), and
new and previously published whole-rock,
major- and trace-element chemistries (Tables
A1 and A2). This active margin sequence has
not been identified previously as such but is
laterally traceable as a ~2500 km long persistent horizon, albeit in detached outcrops, right
from the NW Himalayan sector, through Nepal
and Bhutan, into NE India (Fig. 1). We discuss
implications of our interpretation for correlating
Himalayan stratigraphy, and also for interpreting possible geodynamic scenarios related to the
ca. 1800 Ma Columbia supercontinent.
GEOLOGIC SETTING AND
STRATIGRAPHY
Richards et al. (2005), Robinson et al. (2006),
Upreti (1999), and McQuarrie et al. (2008) provide recent accounts and reviews of the Lesser
Himalayan lithologies and radiometric ages
from northwestern India, western Nepal, central
Nepal, and Bhutan, respectively (Fig. 2). The
following stratigraphic descriptions are based
on their discussion and references therein (especially Stöcklin, 1980; Valdiya, 1980; Gansser,
1983; Bhargava, 1995; Colchen et al., 1986;
Schelling, 1992; DeCelles et al., 2001). In India,
lower Lesser Himalayan rocks are variously referred to as the Jutogh metasediments, Munsiari
Formation or Group, Jaunsar and Damtha
Group, and Rampur Formation or Group
(within the Rampur window). In this paper,
we correlate all these rocks based on lithology and age, and generically refer to them as
“Munsiari.” In Nepal, this interval corresponds
with the lower Nawakot Group, which is further
subdivided, most notably into the Kushma and
GSA Bulletin; March/April 2010; v. 122; no. 3/4; p. 323–335; doi: 10.1130/B26587.1; 9 figures; 3 tables; Data Repository item 2009191.
For permission to copy, contact [email protected]
© 2009 Geological Society of America
323
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11
STDS
MCT
90 °
?
9
10
88 °
7 8
86 °
DS
82 °
ST
6
28°
80°
5
EG
Si
S.I.
75°E
M
AD
D
Ba
B
India
90°E
Bangladesh
15°N
72 °
SP
30°N
Nepal
n
ta
ak
is
P
324
Thrust Fault
Normal Fault
78 °
76 °
74 °
32°
A
34°
1
G
MC
T
4
S
P
30°
M
2
38°
MBT
84 °
Proterozoic-Paleozoic
Greater Himalayan
Sequence
Proterozoic - Phanerozoic
Lesser Himalayan Sequence
Lesser Himalayan Sequence
3 Early Proterozoic dates,
3
BT
M
Figure 1. Geographic and generalized geologic map of the Himalaya showing distribution of Precambrian cratons, major rock types, and
structures (after Stein et al., 2004; Yin, 2006; French et al., 2008). Abbreviations for Indian cratons are: AD—Aravalli-Delhi belt, B—
Bundelkhand craton, Ba—Bastar craton, D—Dharwar craton, EG—Eastern Ghats belt, M—Marwar craton, Si—Singbhum craton,
S.I.—South Indian cratons (Madras, Madurai, Nilgiri, and Trivandrum blocks), SP—Shillong Plateau and Mikir Hills. Dated rocks
that we interpret as components of a ca. 1830 Ma arc include: 1—Besham gneiss, Shang orthogneiss, Kotla Complex; 2—Iskere gneiss;
3—Rampur Complex; 4—Munsiari gneisses and Wangtu gneiss; 5—Amritpur Complex; 6—Kuncha Formation “metasediments”;
7—Ulleri augen gneiss; 8—Kuncha Formation “metasediments”; 9—“Ulleri gneiss”; 10—Granitic orthogneiss; 11—felsic gneiss and
Daling Formation “metasediments”; 12—Bomdila gneiss; S—Sutlej, India; P—Pabar Valley region, India; M—Mori, India; G—
Garhwal region, India. Chronologic details and references are provided in Table 1. STDS—South Tibetan Detachment System; MCT—
Main Central Thrust; MBT—Main Boundary Thrust. Quotes indicate ambiguity in the rock designation.
12
96 °
92 °
94 °
Proterozoic Mesozoic Tethyan
Tertiary Siwaliks
Neogene and
Quaternary
Cenozoic arcs and
Tibetan terranes
98 °
Kohn et al.
Ranimata Formations in the west, the Kuncha
Formation in central Nepal, and the Tumlingtar
Group in the east. The Kushma Formation is a
nearly pure quartzite, stratigraphically below
the Ranimata Formation, and not obviously
associated with igneous rocks. The Kuncha and
Ranimata Formations are dominated by clastic
material but contain minor amphibolites and
a felsic orthogneiss that is usually correlated
with the Ulleri augen gneiss, although neither
the Ulleri nor other felsic orthogneisses ubiquitously bear augen structure. In Bhutan, basal
quartzite (Shumar Formation) is overlain by
chloritic phyllite and quartzite (Daling Formation), which additionally contains sheared
orthogneiss. The Shumar, Daling, Kuncha,
Kushma, Ranimata, Tumlingtar, and Munsiari
units are all part of the lower Lesser Himalayan
sequence. The boundary between “upper” and
“lower” Lesser Himalayan rocks is not agreed
upon but is usually placed below units that exhibit significant carbonate components (Fig. 2).
The Munsiari has been described as containing garnet-staurolite-mica schist, quartzite,
marble, calc-silicate, mafic amphibolite and
graphitic schist, with occasional quartzofeldspathic gneiss (Richards et al., 2005). The
Ranimata, Kuncha, and Daling Formations are
generally described as chloritic phyllite with
scattered quartzite, and either sparse dioritic intrusions (Ranimata Formation: Robinson et al.,
2006; Kuncha Formation: Stöcklin, 1980), or
mylonitized orthogneiss (Daling Formation:
McQuarrie et al., 2008). We found at least one
felsic metavolcanic rock in the Kuncha Formation (Fig. 3), and Richards et al. (2006) interpreted a rock from the Daling Formation as a
metarhyolite. Because many sections are dominated by clastic material, previous work has
emphasized sedimentary, not igneous origins,
essentially describing the units as dominated by
metashale, metasandstone, or metacarbonates,
with intercalated but uncommon igneous bodies of unspecified origins. For example, rocks
in NW India are sometimes referred to as the
Jutogh metasediments, and clastic sedimentary
protoliths are always listed first among rock
types in formation descriptions. Whereas previous workers did faithfully record the occurrence
of igneous rocks, and many Indian geoscientists
proposed various tectonic scenarios based on
igneous geochemistry (e.g., Bhat et al., 1998;
Ahmad et al., 1999; Ahmad, 2008), such rocks
have been largely ignored in interpretations of
the configuration of the northern Indian margin.
Although not a focus of this study, carbonateand graphite-rich units of the upper Lesser
Himalayan sequence are worth discussing for
stratigraphic and tectonic context. In Nepal,
these rocks are commonly considered early
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A Lesser Himalayan arc
to middle Proterozoic in age, and unconformably overlain by upper Paleozoic to Cenozoic
rocks of Gondwanan affinity (e.g., Upreti,
1999; DeCelles et al., 2001). In contrast, upper
Lesser Himalayan rocks including carbonates
and associated quartzites are reported to contain ~500–600 Ma fossils in NW and NE India
(Tewari, 2001; Azmi and Paul, 2004; Hughes
et al., 2005), and lower Paleozoic zircons and
isotopic signatures in Bhutan (Long et al.,
2008; McQuarrie et al., 2008). When correlated
into Nepal, these observations imply that the
upper Nawakot is more likely late Proterozoic
to lower Paleozoic, and that a major hiatus or
highly condensed section occurs somewhere
within the Nawakot Group (Azmi and Paul,
2004; Hughes et al., 2005). Such a break would
separate early Proterozoic, ca. 1830 Ma rocks
(at least Shumar, Daling, Kuncha, Ulleri,
Kushma, Ranimata, and Munsiari units), from
~500–600 Ma rocks that are dominated by graphitic slate and carbonate. Definitive ages have
not been recovered from upper Lesser Himalayan rocks in Nepal; therefore, the correlation remains tentative. Note that Myrow et al.
(2003) proposed contemporaneous deposition
of Lesser Himalayan, Greater Himalayan, and
lower Tethyan rocks, and if correct, their model
applies only to the upper section of the Lesser
Himalayan sequence (Richards et al., 2005).
The exact location of the inferred ca. 1 Ga
unconformity or condensed section remains
unknown because several unconformities have
been proposed above and below the level of
the Galyang, Nourpul, and Blaini Formations
(Fig. 2). Azmi and Paul (2004) proposed that the
Blaini Formation is of Vendian age (ca. 600 Ma)
because it is associated with the overlying
10
Latest Proterozoic– Early to middle
lower Paleozoic Proterozoic (?)
(India, Bhutan)
(Nepal)
?
4
?
Rautgara/Berinag/Sangram/Fagfog
Lower Nawakot
Jaunsar/Damtha
2
Blaini/Galyang/Nourpul
Upper LHS
6
Dominantly
Calcic and graphitic
lithologies
Ranimata - Kuncha Munsiari - Daling
(ca. 1830 Ma)
Ulleri
Lower LHS
8
Upper Nawakot
Mussoorie/Tejam
Gondwanan and
other Cenozoic rocks
Stratigraphic thickness (km)
TABLE 1. AGE CONSTRAINTS FROM LOWER LESSER HIMALAYAN ROCKS
Area Age (Ma)
Rock
Reference
1880 ± 24*
1
Besham gneiss
Treloar and Rex (1990)
1864 ± 4*
Shang orthogneiss
DiPietro and Isachsen (2001)
1836 ± 1*
Kotla Complex
DiPietro and Isachsen (2001)
1850 ± 14*
2
Iskere gneiss
Zeitler et al. (1989)
1904 ± 70†,#
Rampur Complex
Frank et al. (1977)
3
1860 ± 60†
Rampur Complex
Trivedi et al. (1985)
1840 ± 16*
Rampur Window orthogneiss Miller et al. (2000)
1820 ± 19*,** Rampur Window metabasalts Miller et al. (2000)
1866 ± 10*
Wangtu orthogneiss
Singh et al. (1994)
1866 ± 64†
Wangtu orthogneiss
Rao et al. (1995)
1866 ± 6*
Wangtu orthogneiss
Richards et al. (2005)
1797 ± 19*
Jutogh leucogranites
Chambers et al. (2008)
†
1907 ± 91
4
Ramgarh basalts
Ahmad et al. (1999)
1870 ± 7*
Ramgarh Complex
Celerier et al. (2008)
1865 ± 3*
Ramgarh Complex
Celerier et al. (2008)
1856 ± 10*
Ramgarh Complex
Celerier et al. (2008)
†
1865 ± 60
Amritpur Complex
Trivedi et al. (1984)
5
§
1880 ± 40
Amritpur Complex
Varadarajan (1978)
6
1850–1875* Detrital Zrc from Kuncha Fm. DeCelles et al. (2000)
,††
1840 ± 30*
“ Ulleri” augen gneiss
DeCelles et al. (2000)
1840 ± 40*
“ Ulleri” augen gneiss
7
Celerier et al. (2008)
Ulleri augen gneiss
Deniel (1985)
1700–1800
1780 ± 23*
Ulleri augen gneiss
This study
8
ca. 1870*
Detrital Zrc from Kuncha Fm. Parrish and Hodges (1996)
1878 ± 11*
“ Ulleri” augen gneiss
This study
1877 ± 11*
Lower LHS metatuff
This study
1780 ± 23*
9
Granitic orthogneiss
This study
1791 ± 12*
Granitic orthogneiss
This study
1832 ± 23*
Granitic orthogneiss
This study
1795 ± 8*
Detrital zrc from pelitic schist This study
1812 ± 3*
GHS (?) orthogneiss
Liao et al. (2008)
“ Ulleri” augen gneiss
10
>1600
Upreti et al. (2003)
1760 ± 70*
11
Felsic gneiss
Daniel et al. (2003)
1790 ± 30*
Metarhyolite
Richards et al. (2006)
1750–1850* Detrital zrc from Daling Fm.
Richards et al. (2006)
1896 ± 16*
Orthogneiss, Daling Fm.
Long et al. (2008)
1743 ± 4*
12
Bomdila augen gneiss
Yin et al. (2009)
1747 ± 7*
Augen gneiss
Yin et al. (2009)
1800–1900† Bomdila augen gneiss
Bhalla and Bishui (1989), Dikshitulu et al. (1996),
Rao (1998)
*Zircon U-Pb or Pb-Pb age.
†
Rb-Sr whole-rock age.
§
K-Ar age.
#
Recalculated for λ(87Rb) = 1.42 × 10– 11 a– 1.
**Recalculated from raw data reported in Miller et al. (2000): age is weighted mean; uncertainty is
weighted scatter of data about the mean.
††
Recalculated from raw data reported in DeCelles et al. (2000).
Abbreviations: Fm.—Formation; GHS—Greater Himalayan sequence; LHS—Lesser Himalayan
sequence; Zrc—zircon.
Kushma-Shumar
0
Figure 2. Simplified stratigraphic column of
Lesser Himalayan sequence (LHS; Upreti,
1999; DeCelles et al., 2001; Azmi and Paul,
2004), illustrating proposed correspondence
with a continental arc (indicated by patterns)
and ca. 1 Ga unconformity between lower
and upper Lesser Himalayan rocks. Question marks indicate that assignment of the
Nourpul, Galyang, and Blaini Formations to
either ca. 1800 Ma versus 600 Ma ages is uncertain. Note that Stöcklin (1980) places the
upper-lower Nawakot boundary stratigraphically higher within calcareous rocks.
Precambrian–Cambrian Krol Formation in the
Tejam Group. The Blaini has been similarly correlated with Neoproterozoic (ca. 650 Ma) rocks
in China (Jiang et al., 2003; Zhang et al., 2008).
Azmi and Paul (2004) further correlated the
Blaini and Nourpul Formations based on general lithostratigraphy. But according to DeCelles
(2008, personal commun.), the Nourpul Formation contains ca. 1770 Ma mafic rocks, implying that the Nourpul should be grouped with
lower Nawakot rocks, and that any major unconformity must occur at higher stratigraphic levels.
EVIDENCE FOR AN ARC ORIGIN
Field and Textural Observations
Field relationships and microscopic textures
of many Munsiari rocks in NW India indicate
igneous origins, both intrusive and extrusive.
For example, granite porphyries crop out in
the Pabar region (Figs. 3A and 3B), as well as
coarse-grained felsic gneiss that we interpret as
deformed granite (Fig. 3C). Rocks that resemble
(meta)sandstones in hand-sample have typical
igneous whole-rock compositions (Table A1)
and reveal textures that are equally consistent
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Kohn et al.
A
B
10 cm
Finegrained
10 cm
C
10 cm
Porphyritic
D
2 cm
5 cm
E
Quartz
phenocrysts?
F
Metapumice
fragment?
1 mm
G
Schist
H
Metavolcanic
10 cm
2 cm
Figure 3. Field and thin-section photographs of felsic rocks from Munsiari rocks, northwestern India (A–F) and from Kuncha Formation, central Nepal (G and H). (A–C) Granites
and granitic gneiss in Pabar region, India. (D–F) Hand sample and photomicrographs in
plane-polarized light and cross-polars of fine-grained “sandstone,” here interpreted as a
metamorphosed altered tuff that originally contained quartz phenocrysts (knots in inset of
D; coarse quartz in E and F), and pumice fragments (light streaks in D; compositionally distinct domains in E and F). (G) Felsic metavolcanic rock associated with chlorite-rich schist,
Kuncha Formation, Langtang region, Nepal. Zircon rims from this sample give an age of
ca. 1880 Ma. (H) Augen gneiss in Langtang region, Nepal that is correlated with Ulleri
augen gneiss. Zircon rims from this sample give an age of ca. 1880 Ma.
326
with a tuff protolith, originally containing quartz
phenocrysts and stretched and flattened pumice
fragments (Figs. 3D–3F). Some Lesser Himalayan rocks from central Nepal retain feldsparrich metaigneous chemistries, mineralogies, and
textures, and reflect both volcanic and intrusive
felsic phases (Figs. 3G and 3H).
Mafic igneous rocks also occur in the lower
Lesser Himalayan sequence, commonly as
isolated strata that may be intercalated with
metasedimentary rocks such as quartz arenites,
but they are also associated with felsic metaigneous rocks. One key observation is that
minor but widespread chlorite schist in NW
India exhibits textures consistent with a mafic
volcanic or sill protolith that has been hydrated
and metamorphosed. In rare instances, chlorite
schist is localized at the margins of mafic amphibolites (Fig. 4A). We interpret this schist as
the tops of flows, altered and hydrated either
soon after deposition or during metamorphism as a result of enhanced fluid flow along
lithologic boundaries. In other instances, mmdiameter white spheroids are hosted in a mafic
matrix (Fig. 4B). In thin section, the spheroids
are dominated by plagioclase with subordinate
quartz, carbonates, and chlorite (Figs. 4C and
4D). The matrix contains abundant coarsegrained plagioclase. We interpret the spheroids
as metamorphosed amygdules and the matrix as
metamorphosed porphyritic basalt. More generally, we find a progression of variably hydrated
mafic assemblages from chlorite-rich through
amphibole-rich schist, with variable retention of
original porphyritic textures (Figs. 4E and 4F).
The stratigraphic and structural relationships of felsic and mafic rocks with surrounding
metasedimentary rocks help constrain possible
genetic and tectonic interpretations. Felsic plutonic bodies (Ulleri gneisses) in Nepal are intercalated with “tuffaceous” metasedimentary
rocks (Le Fort, 1975; Le Fort and Raï, 1999).
The Ulleri and surrounding schists share fabrics,
so must have been codeformed, presumably during the late Cenozoic (Le Fort, 1975). Contacts
have been described both as gradational with
adjacent schists and quartzites (Le Fort, 1975;
Le Fort and Raï, 1999) and as obliterated by
later deformation (Yin et al., 2009). So, whereas
some have proposed that the lower contacts are
thrust faults and that the Ulleri represents Indian basement (Gansser, 1964; Yin, 2006; Yin
et al., 2009), others interpret the felsic gneisses
to reflect either syngenetic porphyritic extrusive
rocks (Le Fort, 1975; Le Fort and Raï, 1999),
or intrusions into a dominantly sedimentary sequence (e.g., DeCelles et al., 2000), with transformation to gneisses during later deformation.
Along-strike variations are evident in the
abundances of igneous components in the lower
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A Lesser Himalayan arc
0.5m
A
Lesser Himalayan sequence. For example, plutonic rocks are relatively common in the Munsiari of NW India but are relatively rare in the
Kuncha and Ranimata Formations of Nepal,
which are instead dominated by chlorite- and
feldspar-rich schist. Thus, any putative arc in
Nepal must be either largely buried under overlying thrusts (e.g., the Main Central Thrust),
or dominated instead by volcanic rocks or volcanogenic sediments whose postdeformational
and postmetamorphic physical appearance now
masquerades as deformed and metamorphosed
passive margin sediments.
B
Chlorite schist
Amphibolite
Amphibolite
Amphibolite
5 cm
C
Plagioclase
phenocrysts
Major- and Trace-Element Geochemistry
and Zr-Saturation Temperatures
D
Metamorphosed
amygdules
1 mm
E
F
Plagioclase
phenocrysts
1 mm
1 mm
Figure 4. Field and thin-section photographs of mafic rocks from Munsiari rocks, northwestern India. These observations suggest that widespread chlorite schist that we observed
in the field was, in fact, sourced by basaltic material. (A) Multiple amphibolites with chlorite
schist at top, interpreted as possible basalt flows. Mori region, NW India. (B) Metamorphosed amygdaloidal basalt. Most white spheroids are plagioclase, but some also contain
carbonate, quartz, or chlorite. These are interpreted as metamorphosed zeolitic infillings.
Pabar region, NW India. (C–F) Photomicrographs of greenschist- and amphibolite-facies
metabasalt from the Pabar region, NW India, illustrating relict igneous textures and progressive development of chlorite schist from a basaltic precursor. (C and D) Metamorphosed
amygdaloidal basalt in plane polarized light and in crossed polars. Amygdules in this rock
are now dominated by plagioclase, which probably represents the metamorphosed equivalent of original infilling zeolites. Amygdules in other rocks contain carbonate + plagioclase ±
chlorite, or quartz. Matrix silicate assemblage is plagioclase + hornblende + chlorite + biotite + epidote + quartz. (E) More deformed and hydrated amygdaloidal metabasalt, showing
sheared relict amygdule and relict porphyritic feldspar texture. Matrix silicate assemblage
is plagioclase + chlorite + biotite + epidote + titanite + quartz. (F) Chlorite schist, retaining
a few relict porphyritic feldspars. Silicate assemblage is plagioclase + chlorite + biotite +
titanite + quartz.
The overall mineralogy and major-element
chemistry of some lower Lesser Himalayan
“sediments” in NW India and Nepal is consistent
with a volcanogenic or even volcanic origin. In
Nepal, lower Lesser Himalayan schist is generally graphite-poor, uniformly feldspar-rich, and
contains low-Al assemblages (Catlos et al., 2001;
Kohn, 2008). Some samples from the Munsiari in
NW India that were identified as metasedimentary rocks have compositions similar to felsic
volcanic rocks (Table A1), specifically exhibiting
much lower Fe contents (<5.5 wt% Fe2O3) and
K/Na ratios (<1.7) than average pelites (>5.7 wt%
Fe2O3 and >2.0). In fact, these compositions
closely match those of dacite and rhyolite.
Trace-element geochemistry further supports
either an arc or collisional setting. Mafic rocks
have been variously interpreted as arc, rift, or
flood basalt magmas (Bhat et al., 1998; Ahmad
et al., 1999; Miller et al., 2000; Ahmad, 2008).
However, high large ion lithophile elements
(LILE), low TiO2, and negative Ta and Nb anomalies are more consistent with an arc (Miller
et al., 2000). Discrimination diagrams (Pearce
et al., 1984) were considered for Y, Nb, and Rb
in felsic rocks because data for these elements
are available for many samples. For Nb versus
Y, data plot within the volcanic arc and syncollisional fields, implying that within-plate plume
or ridge settings are unlikely (Fig. 5A). For Rb
versus Nb + Y, data plot near the triple point of
the fields for within-plate, volcanic arc, and collisional granites (Miller et al., 2000; Fig. 5B).
These interpretations should be viewed with caution because Nd-model ages (Miller et al., 2000;
Richards et al., 2005) exceed crystallization
ages, perhaps indicating crustal contamination
that would bias trace-element compositions,
particularly for Rb, which is generally viewed
as more mobile than Y and Nb. Nonetheless,
Zr-saturation thermometry (Watson and Harrison, 1983) for felsic gneiss and some possible
volcanic rocks indicates magmatic temperatures
Geological Society of America Bulletin, March/April 2010
327
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Kohn et al.
15
100
Zr-saturation temperatures
Leucogranites
A
Within plate
12
Volcanic (?)
Nb (ppm)
Volcanic arc and
syn-collisional
Number
Shang
MJ156
HF102/90
10
0
600
1
10
Rb (ppm)
Y (ppm)
B
100
1000
AR01-10c
Shang
Metavolcanic
(?) rocks
100
Volcanic arc
Within plate
Ocean ridge
10
1
10
Y+Nb (ppm)
100
1000
Figure 5. Trace-element data (Rb, Y, Nb) for Lesser Himalayan felsic igneous and metavolcanic (?) rocks plotted on discrimination diagrams (Pearce et al., 1984), showing best agreement with either volcanic arc or syncollisional origin. Compositions of Langtang (LT)
samples, leucogranites from the Sutlej Valley, and possible metavolcanic rocks are slightly
more consistent with an arc. Some specific outliers are identified.
of 800 ± 50 °C (Table A2; Fig. 6), again consistent with relatively wet melting at low temperature in an arc or collisional setting rather than
the higher temperatures anticipated for a flood
basalt or rift setting. Chambers et al. (2008) identified even lower temperatures for ca. 1810 Ma
leucogranites in northwestern India, and ascribed these to wet crustal melting. Correction
of Zr contents for any zircon inheritance would
lower calculated temperatures, further underscoring low magmatic temperatures.
328
700
750
800
850
900
Figure 6. Zirconium saturation temperatures from Paleoproterozoic felsic gneisses,
showing typical (maximum) temperatures
of ~800 °C. Data from Le Fort and Raï
(1999), Richards et al. (2005), Miller et al.
(2000), Chambers et al. (2008), Sharma and
Rashid (2001), and this study.
MJ156
HF102/90
LT &
Leuco
650
Temperature (°C)
Multiple sources
Sharma and Rashid (2001)
Syn-collisional
6
3
Ocean ridge
1000
9
Metavolcanic
(?) rocks
LT &
Leuco
1
Gneisses
Geochronology
Published dates for mafic rocks, orthogneiss,
and crosscutting leucogranite generally fall between 1810 and 1870 Ma (Table 1). These ages
are distributed across the length of the Himalaya, and their relatively limited age range and
common geochemical characteristics imply
a single coeval origin for these igneous rocks.
The curvilinear distribution of these coeval
rocks may reflect their original orientation, al-
though later Cenozoic deformation could have
changed their distribution. New data from
Arunachal from felsic gneisses appear younger
(ca. 1745 Ma; Yin et al., 2009) than most other
lower Lesser Himalayan ages. These ages could
represent a younger phase of the same magmatic
event we propose across the Himalaya. Alternatively ages of plutonic rocks in Bangladesh are
as old as 1720–1730 Ma (Ameen et al., 2007;
Hossain et al., 2007), and a discrete younger
magmatic event may be regionally significant.
It is important to note that Sharma and Rashid
(2001) recognized the common ages for some of
these rocks along the Himalaya and argued for
formation in some consistent tectonomagmatic
setting. However, they did not propose a geodynamic setting for these rocks, or otherwise
offer any other genetic explanation.
New zircon geochronologic data (Table 1;
Fig. 7; see GSA Data Repository1) further support the occurrence of a Paleoproterozoic arc
in Nepal but expand the possible age range to
1780–1880 Ma. Sampling and analytical methods are described in the Appendix, and results
are shown on standard Concordia diagrams
(Fig. 7). These zircons are interpreted as igneous, rather than metamorphic, based on high
Th/U ratios (see data repository; Hoskin and
1
GSA Data Repository item 2009191, Zircon
U-Pb isotopic data and XRF whole-rock data from
possible igneous rocks from India and Nepal, compilation of zircon U-Pb and Pb-Pb ages from the Indian
craton, and locations of samples from Nepal, is available at http://www.geosociety.org/pubs/ft2009.htm
or by request to [email protected].
Geological Society of America Bulletin, March/April 2010
0.0
0.1
0.2
0.3
1.86
1.88
M
0.0
0.0
1.0
0.0
207Pb/235U
0.0
207Pb/235U
24±44
M
D
SW
–8±48
–8±59
1000
600
=
=
8
1.
600
D
SW
14
0.
1400
2.0
D
SW
M
1884±18
=
0.
1.0
3.7±1.3
17
D
SW
M
600
AS01-5
0.0
1400
1777±13
1800
1780±23
0.1
1.86 1.86
1.87
1.78
1.80
AS01-5
1.0
0.
21
1.88
1.87
LT01-44a
1.87
LT01-102
1.0
D
=
1800
1000
0.2
0.3
0.4
0.0
M
–15±36
SW
2±11
0.
600
M
D
=
38
SW
600
1000
1400
Cores not used in regression
=
3
4
0.
1000
1800
0.0
1800
74±89
=
2600
0.
18
1000
1800
8.0
42 rims
3 cores
1877±11
2200
1878±11
1400
1800
1±59
D
SW
M
600
LT01-102
0.0
1400
1791±12
16.0
0
1600
5
10
15
20
25
3000
LT01-44a
4.0
1.80 1.78
1.87
2.00
AR01-4c
24.0
2400
3200
Age(Ma)
1.79
1.76
6.0
0.0
0.2
0.4
0.6
0.8
1.79
2.23
1.81
AR01-4c
1.80
1.78
1.79
Cores
Rims
3400
1.79
1.81
1.77
AR01-40b
1795±8
1.80
AR01-15
1800
~1880±10
1.79
1.80
207Pb/235U
2.0
1400
1832±23
AR01-10c
Relative Probabiliy
Figure 7. New U-Pb zircon ages from Lesser Himalayan rocks from central and eastern Nepal, showing inferred crystallization ages ranging from ~1780 to ~1880 Ma. Note
that sample AS01-5 is from type Ulleri augen gneiss, and gives a significantly younger age than other felsic intrusions (e.g., LT01-102) that are correlated with it.
206Pb/238U
Data not used in regression
MS
206Pb/238U
AR01-15
4
AR01-40b
0.3
Geological Society of America Bulletin, March/April 2010
WD
=
Data used in regression
Number
0.4
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A Lesser Himalayan arc
329
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Kohn et al.
DISCUSSION
Most Evidence Points to an Arc
Several lines of evidence support an arc interpretation for the origin of many lower Lesser
Himalayan rocks. Field observations reveal a
widespread felsic igneous component within the
Ranimata, Kuncha, and Daling Formations across
330
20
18
LHS
16
Indian Craton
LHS arc
Zircon ages
14
12
Number
Schaltegger, 2003) and elongate morphologies
(Fig. 7; Corfu et al., 2003). Many zircons show
evidence for major recent Pb loss that is either
modern or Himalayan (<~40 Ma) or both. Yet
considering analytical errors and the antiquity of
these rocks, young Pb loss does not significantly
obscure crystallization ages. For samples from the
Arun Valley in eastern Nepal, zircon ages range
between 1780 and 1830 Ma, overlapping but
somewhat younger than most published ages of
other igneous rocks across the orogen (Table 1).
Type Ulleri augen gneiss from Ulleri, Nepal
(AS01-5), gives a similarly young, preferred
crystallization age of ca. 1780 Ma, but with a
likely inherited component of ca. 1880 Ma.
Sample LT01-102, from Langtang, Nepal, is
correlated with Ulleri augen gneiss, yet gives
an age of ca. 1880 Ma—clearly older than type
Ulleri, but indistinguishable from the age we
infer for a felsic metavolcanic rock intercalated
with Kuncha schist (LT01-44a). Note that we
focused on analyzing zircon rims, as determined
from cathodoluminescence images, but in
LT01-44A we also analyzed numerous, presumably inherited cores. Core analyses show a radically different pattern compared to rim analyses:
whereas rims strongly cluster at ca. 1880 Ma,
cores range widely from 1880 to 3300 Ma.
Putatively detrital zircons in the lower Lesser
Himalayan sequence across Nepal show a preponderance of 1800–1900 Ma ages, implying
abundant primary magmatic material of that age
(e.g., DeCelles et al., 2000, 2004). In at least
two instances, metasedimentary rocks yielded a
single zircon age peak in that range (DeCelles
et al., 2000, 2004). Similarly, we found a pronounced age peak in pelitic schist from Arun
(AR01-4), although our analyses are strongly
biased toward rims. Possibly some of these
sedimentary rocks are of volcanic origin or have
a major volcanogenic component, with zircons
derived from local contemporaneous sources.
These zircons probably did not derive from cratonal India, which exhibits a distinct age gap between 1750 and 2450 Ma (Parrish and Hodges,
1996; Fig. 8). The only ages from the Indian
craton that overlap the age distribution of Lesser
Himalayan rocks are for a pluton in the AravalliDelhi belt, and for a suite of mafic dikes in the
Bastar craton (Figs. 1 and 8).
10
8
6
Bastar
Craton
mafic
dikes
Aravalli
4
2
0
800
1200
1600
2000
2400
2800
3200
3600
Age (Ma)
Figure 8. Compilation of zircon U-Pb and Pb-Pb ages of igneous rocks from the Indian craton (black bars), showing pronounced peaks at ca. 2500 and ca. 1700 Ma, but paucity of ages
at ca. 1800 Ma. Zircon ages from Table 1 shown as white bars. Age of proposed arc overlaps
with ages of only two cratonal rocks: a ca. 1885 Ma mafic dike complex in the Bastar craton
and a ca. 1850 Ma intrusion in the Aravalli belt. Relatively young ages of Lesser Himalayan
igneous rocks in Arunachal (ca. 1745 Ma; Yin et al., 2009) overlap with tail of a ca. 1700 Ma
peak (e.g., data from Bangladesh and the Aravalli belt). The Arunachal gneisses may represent rocks formed in a different setting, or continuation of arc activity to ca. 1750 Ma.
the ca. 1000 km breadth of the Nepal (Le Fort
and Raï, 1999) and Bhutan sectors (Gansser,
1983). Metamafic rocks are even more widespread (Stöcklin, 1980; Valdiya, 1980; Gansser,
1983; DeCelles et al., 2001; Robinson et al.,
2006; McQuarrie et al., 2008), both as amphibolite and as mafic chlorite schist (Fig. 4). These
observations and interpretations differ markedly
from previous studies in northwest India (e.g.,
Vannay and Grasemann, 1998; Richards et al.,
2005) but agree better with reports from western Nepal (e.g., DeCelles et al., 2001; Robinson et al., 2006). Although some sections do
contain abundant schists of sedimentary origin,
we found abundant intrusive and volcanic rocks
in the Munsiari of NW India, especially in the
Pabar region (Figs. 3 and 4). The occurrence of
these geographically widespread igneous rocks
along a curvilinear belt further favors an active,
rather than passive margin interpretation. Finally,
distributed igneous rocks could also form in a
rift environment, but the low Zr-saturation temperatures in felsic rocks (Chambers et al., 2008;
this study), the trace-element geochemistry of
mafic and felsic rocks (Miller et al., 2000; this
study), and the protracted (ca. 100 Ma) magma-
tism are more consistent with either an arc or a
collisional origin.
An arc may also explain some lithologic
changes along strike. In northwest India, felsic intrusive and felsic to mafic volcanic rocks
can dominate the Munsiari, for example in the
Pabar region (Figs. 1, 3, and 4), whereas in
Nepal such obvious igneous rocks are relatively
rare at the same stratigraphic level. Instead, we
hypothesize that the abundant schist in Nepal
and Bhutan has a major volcanic or volcaniclastic component, based on low-Al, feldsparrich mineral assemblages and mesoscopic and
microscopic textures (Figs. 3 and 4). These
differences along strike could simply reflect
geographic variations in magmatic intensity, differences in exposure through the arc sequence,
or both. In India and Bhutan, a thick, lower
Lesser Himalayan sequence of ca. 1830 Ma arcrelated rocks overlain by a thick, upper Lesser
Himalayan sequence of post–ca. 600 Ma rocks
further underscores the fundamental stratigraphic disparity highlighted by Azmi and Paul
(2004) and Hughes et al. (2005). Altogether our
model implies that a Paleoproterozoic arc forms
the stratigraphic base to the northern edge of the
Geological Society of America Bulletin, March/April 2010
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A Lesser Himalayan arc
A 1780–1880 Ma arc along the northern margin of India may help resolve debate about the
configuration of the ca. 1800 Ma supercontinent
Columbia. Three basic models have been proposed for the relative placements of India, North
America, and East Antarctica (Fig. 9). Rogers and
Santosh (2002) and Zhao et al. (2004) sandwich
East Antarctica between India and North America, leaving the northern edge of India as a passive margin, in agreement with many views of
the origins of the Lesser Himalayan sequence. In
contrast, Hou et al. (2008) place India directly
adjacent to North America, with a continuous
subduction zone that includes the northern edge
of India and parts of East Antarctica. Although
some models could perhaps be modified to include subduction along the northern edge of
“Detrital” Zircon Analysis
If correct, our model has additional implications for the collection and interpretation
of detrital zircon ages. Many laser ablation–
inductively coupled plasma–mass spectrometer
(LA-ICP-MS) studies of presumed detrital zircons focus on analyzing cores. This approach
can minimize potential Pb-loss problems that
we clearly encountered in several of our analyses. It further assumes that the resulting age
spectrum is diagnostic of the source material
and, for the youngest retrieved ages, limits
the maximum depositional age (e.g., DeCelles
et al., 2001). While we fully endorse detrital
zircon dating in such endeavors, we do note that
the data distribution for LT01-44A is highly
skewed for cores compared to rims. For rims,
44 of 54 analyses yielded indistinguishable
ca. 1880 Ma ages that we interpret as a crystallization age. In contrast, only four core analyses indicated this age, whereas the remaining
16 core analyses ranged between ~2100 and
3300 Ma. Had we analyzed only cores, as is
A
B
C
EA
v
v
v
NC
NA
vvvv
EA
NC
NA
vv
v
vv
Several studies have interpreted the orthogneisses in the lower Lesser Himalayan sequence (e.g., “Ulleri”) to represent Indian
cratonal basement (e.g., Gansser, 1964; Ray
et al., 1989; Richards et al., 2005; Yin, 2006;
Yin et al., 2009). In this model lower Lesser
Himalayan sediments were either deformed
and metamorphosed during the early Proterozoic, or deposited unconformably on crystalline
plutonic rocks and gneisses. In principle, the
crystalline rocks could represent new crustal
additions at ca. 1830 Ma, i.e., the roots of a
Proterozoic arc, or alternatively recrystallized
Archean material, i.e., older plutons and sediments that were deformed and metamorphosed
during a ca. 1830 Ma collisional event (e.g.,
see Fig. 9 of Richards et al., 2006). Such a
model does have some supporting evidence:
relict zircon cores are as old as 3300 Ma, and
trace-element geochemistry for many plutonic
rocks is as consistent with a collisional origin
as with an arc. Two key observations, however,
do not favor either a wholly plutonic origin for
1800 Ma igneous rocks or collisional reworking
of older materials.
(1) The oldest metamorphic age (for allanite) yet recovered from Lesser Himalayan
rocks is less than 500 Ma (Catlos et al., 2000).
For example, garnet ages are 7–11 Ma (Vannay
et al., 2004), and monazite ages are as young as
ca. 3 Ma (Catlos et al., 2001, 2007; Kohn et al.,
Reconstruction of the Columbia
(ca. 1800 Ma) Supercontinent
India, Hou et al.’s model is the only one proposed so far that conforms to our interpretation
of the Lesser Himalayan sequence. We emphasize that several other competing models have
been proposed for the configuration of Columbia that infer quite different positions than Hou
et al. (2008) for North America, Baltica, Australia, etc. (Krapez, 1999; Betts et al., 2008;
Bispo-Santos et al., 2008; Payne et al., 2009).
Our interpretation for India in no way validates
or refutes these other models. It does, however,
suggest that ≥2500 Ma Indian cratonal provinces
should not be extrapolated to the north onto other
continents at ca. 1800 Ma, because this margin
appears to have been active at that time.
A
Indian “Basement” and a
Paleoproterozoic Collision?
2004). Thus, there is as yet no direct metamorphic evidence for a Paleoproterozoic collision.
(2) Felsic volcanic rocks are intercalated with
felsic plutonic and mafic volcanic rocks and
yield similar ages as the plutons (Le Fort, 1975;
Le Fort and Raï, 1999; Richards et al., 2005,
2006; this study). A shallow origin for these volcanic rocks is indisputable—in addition to relict
volcanic textures (Figs. 3 and 4), many are intercalated with sedimentary rocks. Thus, igneous
rocks of the lower Lesser Himalayan sequence
cannot represent only the crystalline roots of a
volcanic arc either. The simplest interpretation
is that, although some transposition of contacts
and shearing must have occurred in the Cenozoic, the present intercalation is largely primary.
Thus, the lower Lesser Himalayan gneisses can
be neither assigned to Indian basement nor ascribed to Paleoproterozoic collision.
CE
exposed Indian plate, and provides a stronger
basis for correlating rocks along the Himalaya
and for inferring structure, particularly along
the Main Central Thrust, where Greater and
Lesser Himalayan rocks are juxtaposed. The
arc is mostly preserved as volcanically derived
sediments but with important felsic and mafic
intrusive and volcanic components.
Although the northern edge of the arc, including any accretionary material, is likely
buried beneath the Himalaya and Tibet, some
enigmatic rocks on the Indian craton could perhaps be genetically linked. Specifically, a suite
of mafic dikes in the Bastar craton has recently
been dated at ca. 1885 Ma (French et al., 2008),
at the beginning of the time when we propose
the arc was active. Possibly this dike swarm
reflects thermal disturbances related to arc initiation or, alternatively, to backarc spreading.
Further geochemical analysis of the dikes might
help elucidate their origin(s) and links to Lesser
Himalayan rocks. Note that the long hiatus between deposition of lower and upper Lesser
Himalayan rocks complicates any attempts to
infer postsubduction processes.
NA
Figure 9. Plate reconstructions for ca. 1800 Ma supercontinent Columbia showing different positions for India (shaded); v’s indicate proposed volcanic arc. (A and B) Models of
Rogers and Santosh (2002) and Zhou et al. (2004) showing passive margin for northern India.
(C) Model of Hou et al. (2008) showing subduction zone along northern Indian margin, exactly as we propose. NA—North America; EA—East Antarctica; CEA—Coastal East Antarctica; NC—North China craton. Note that several other models for Columbia do not attempt to
reconstruct India’s position; therefore, our proposed arc does not discriminate among them.
Geological Society of America Bulletin, March/April 2010
331
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Kohn et al.
commonly done, we would still have identified
the youngest, ca. 1880 Ma age, but we would
have missed just what a vast preponderance of
zircon was formed at that time.
Although we presumed this rock was a
metamorphosed tuff based on its mineralogy
and physical appearance—an interpretation
supported by its bulk chemical composition (Table A1)—not all volcanic rocks are so
distinctive after metamorphism, and could be
interpreted and processed as if they were
metasandstones. In that regard, we cannot directly evaluate any possible bias inherent in published zircon age distributions for the Himalaya.
Some workers document primary sedimentary
features both in the field and petrographically,
indicating substantial reworking of detrital
zircons (P. DeCelles, 2009, personal commun.), but details are rarely published. Possibly
some lower Lesser Himalayan rocks could be
metavolcanic with inherited zircon cores, much
as we interpret LT01-44A. If so, the common
>1900 Ma ages obtained from “detrital” zircon
cores represent inheritance, and closer consideration of magmatic rim overgrowths will reveal
an even more pronounced 1830 ± 50 Ma age
peak. Such refined ages might help delineate the
extent of the Paleoproterozoic rocks as well as
the structure of Himalayan thrusts.
CONCLUSIONS
We hypothesize that the basal part of the
Lesser Himalayan sequence represents the edge
of a 1830 ± 50 Ma continental arc, based on
field and textural observations, whole-rock
chemistry, and geochronology. This model
explains the occurrence of so many coeval
igneous rocks distributed widely along the
Himalaya, the chemistry of the metasedimentary and igneous rocks, and the distribution
of zircon ages. Younger, ca. 1745 Ma ages in
Arunachal (Yin et al., 2009) may extend the
duration of the arc by an additional ca. 30 Ma.
These results distinguish among models for the
placement of India in the ca. 1800 Ma supercontinent Columbia. The model of Hou et al.
(2008) is consistent with an active margin
for northern India. Thus, although Hou et al.
(2008) wrote: “Another subduction zone may
have existed…along the northern margin of
the Indian Craton, but has been lost beneath
the Eurasian Plate,” we find no need to hypothesize a “Lost Arc of the Continent”: a Paleoproterozoic arc is present and can be accounted
for. Felsic orthogneisses (“Ulleri”) within the
Lesser Himalayan sequence probably do not
represent Indian basement but rather relatively
shallow intrusions in an arc edifice and associated sedimentary pile.
332
APPENDIX: SAMPLES AND
ANALYTICAL METHODS
Samples that we analyzed for U-Pb zircon ages
were collected from the Annapurna (sample prefix
AS01), Langtang (LT01), and Arun (AR01) regions
of central and eastern Nepal. See the Data Repository
(footnote 1) for maps showing sample locations. Arun
samples were all collected from a granitic orthogneiss
unit sometimes referred to as the Num orthogneiss,
but they are also correlated with the Ulleri augen
gneiss (e.g. Goscombe and Hand, 2000; Goscombe
et al., 2006). Three Arun samples are metagranite,
and one sample (AR01-4) is a pelitic schist dominated
by quartz, muscovite, and biotite. Sample AS01-5 is
from Ulleri augen gneiss from its type locality in the
town of Ulleri, Nepal. Sample LT01-102 is granitic
orthogneiss from the Munsiari thrust sheet, and is correlated with Ulleri augen gneiss based on stratigraphic
position and appearance (e.g. Kohn, 2008). Because
some felsic orthogneisses correlated with Ulleri
augen gneiss give different ages, we refer to them as
“Ulleri.” Sample LT01-44a appeared texturally to be
a metamorphosed felsic tuff (Fig. 3G).
Zircons were separated using standard separation
techniques at Boise State University, and mounted
with age standard FC-1, which has a U-Pb age of
1099 Ma (Paces and Miller, 1993). Zircons were imaged using cathodoluminescence at the Center for
Electron Microscopy and Microanalysis at the University of Idaho, and analyzed for U-Pb and Pb-Pb
ages in the Geoanalytical Laboratory, Washington
State University, using methods described by Chang
et al. (2006). In brief, each sample was ablated using
a 213 nm laser operating with a 30 µm diameter spot
and ~10 J/cm2 fluence. Each spot was accurately plotted on a cathodoluminescence (CL) image, particularly noting whether a core versus a rim was analyzed,
as determined either chemically, spatially, or both.
Ablated material was carried on a He stream to the
source of an Element2 ICP-MS, a single-collector,
magnetic sector instrument. Lead, thorium, and uranium isotopes were measured in low-resolution mode,
ratioed, and corrected for in-run drift. Mass fractionation corrections were based on bracketing analyses
of FC-1. No correction for 204Pb was made because,
within uncertainty, there was no 204Pb after correcting for 204Hg interference (based on measurements of
202
Hg), and because even the raw ratio of mass 206 to
mass 204 typically exceeded 5000, implying a negligible common Pb correction. Ratios of Th/U were
estimated for analyses at least 95% concordant for
235
U-207Pb and 238U-206Pb by assuming concordance
between U-Pb and Th-Pb ages.
Corrected ratios, uncertainties, and ages include a
~1%–2% standardization error based on the scatter
of analyses of FC-1, i.e. the standard deviation, not
standard error. As discussed elsewhere (Chang et al.,
2006; Kohn and Vervoort, 2008), this error assignment
accurately accounts for errors for a single analysis of
an unknown, but it overestimates errors for pooled
data. Consequently, the mean square of the weighted
deviates (MSWD) for age regressions will be erroneously small. This is evident in our data (Fig. 6),
where MSWDs are routinely less than ~0.5—mainly
a result of how we choose to propagate standardiza-
TABLE A1. CHEMISTRY OF METAVOLCANIC (?) ROCKS COMPARED TO
GRANITE, AVERAGE SHALE, AND CA. 1830 MA HIMALAYAN FELSIC ORTHOGNEISSES
Possible metavolcanic rocks
Source
R05
R05
C08
C08
C08
This study
This study
Sample
W50
W54
63ii
71ii
81
SV798
LT01-44a
SiO2
75.89
68.99
68.98
67.84
70.70
74.05
78.67
TiO2
0.20
0.63
0.61
0.72
0.49
0.63
0.35
13.77
14.97
15.91
15.39
14.47
11.32
11.35
Al2O3
1.44
4.25
4.86
5.47
4.74
4.35
2.05
Fe2O3
MnO
0.01
0.03
0.04
0.08
0.25
0.08
0.23
MgO
0.94
2.80
1.51
1.52
1.32
1.70
0.47
CaO
1.25
1.08
1.95
2.62
2.92
1.48
3.09
2.32
1.85
1.85
3.12
2.74
1.49
1.96
Na2O
3.36
4.71
3.73
2.54
2.24
2.99
1.21
K2O
0.09
0.11
0.15
0.17
0.11
0.10
0.10
P2O5
LOI
1.00
1.03
1.41
0.96
0.82
N.D.
N.D.
Sum
100.27
100.46
101.00
100.42
100.80
98.18
99.48
K/Na
0.95
1.68
1.33
0.54
0.54
1.32
0.41
Zr (ppm)
102
167
176
213
149
250
159
Reference compositions of granite, Lesser Himalayan felsic orthogneisses (M00, R05), and shales (PAAS,
NASC, Archean)
Granite
M00
M00
R05
R05
PAAS
NASC
Archean
Sample
HF49/90
HF35/92
W60
W65
73.30
68.20
74.60
72.37
68.55
62.80
64.80
60.95
SiO2
TiO2
0.28
0.60
0.20
0.37
0.63
1.00
0.70
0.62
13.50
13.70
13.40
14.50
15.20
18.90
16.90
17.50
Al2O3
2.30
4.90
1.70
3.12
4.36
6.50
5.66
7.53
Fe2O3
MnO
0.10
0.00
0.03
0.05
0.11
0.06
n.d.
MgO
0.42
0.50
0.30
0.89
0.91
2.20
2.86
3.88
CaO
1.30
2.10
1.00
0.96
1.26
1.30
3.63
0.64
3.20
2.40
2.70
1.87
2.84
1.20
1.14
0.68
Na2O
4.80
5.50
5.10
4.76
4.63
3.70
3.97
3.07
K2O
P2O5
0.20
0.10
0.15
0.18
0.16
0.13
0.61
LOI
0.08
0.50
0.30
1.02
1 . 31
0.10
Sum
99.91
98.70
99.40
100.04
99.92
97.87
99.85
95.58
K/Na
0.99
1.51
1.24
1.67
1.07
2.03
2.29
2.97
Zr (ppm)
240
372
150
148
223
210
200
151
Note: PAAS from Taylor and McLennan (1985); NASC from Gromet et al. (1984); granite and Archean from
Condie (1993); M00—Miller et al. (2000); R05—Richards et al. (2005); C08—Chambers et al. (2008).
Geological Society of America Bulletin, March/April 2010
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A Lesser Himalayan arc
tion errors. Interstandard errors were not included in
our age assignments, and some studies suggest these
may contribute an additional 1%–2% systematic error
(e.g., Chang et al., 2006). Thus, for intercomparisons
of sample ages, an additional ±20–35 Ma systematic
uncertainty should be considered.
Two possible metavolcanic rocks that were analyzed for whole-rock chemistry (Table A1) were
collected in the Pabar, India (SV798) and Langtang,
Nepal (LT01-44a) regions. Whole-rock, major- and
trace-element compositions were measured by X-ray
fluorescence (XRF) at the GeoAnalytical Laboratory,
Washington State University, using standard techniques. The supplemental file (see footnote 1) contains
additional whole-rock compositions, including samples from the Annupurna, Langtang, and Arun areas.
ACKNOWLEDGMENTS
REFERENCES CITED
This material is based upon work supported
by the National Science Foundation under grants
EAR-0337050, EAR-0439733, and EAR-0803549
to MJK, and by Boise State University. We thank
Dr. B.R. Arora, Director, Wadia Institute of Himalayan Geology, Dehradun, India, for providing
logistical support and for granting permission to
undertake this joint American-Indian field study,
and Dr. S. Sensarma (Lucknow) for reviewing the
manuscript. Comments from T. Argles, P. DeCelles,
C. Dehler, D. Evans, N. McQuarrie, and associate
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TABLE A2. ZIRCONIUM-SATURATION TEMPERATURES AND TRACE-ELEMENT
CONTENTS FOR LOWER LESSER HIMALAYAN INTRUSIVE AND VOLCANIC (?) ROCKS
Sample
Zirconium
Rock
Rb, Y, Nb contents
Reference
number
temperature
(ppm)
(°C)
LO202
854
Ulleri augen gneiss
N.D.
(1)
Shang
848
Shang orthogneiss
177, 56, 31
(2)
Kotla
859
Kotla Complex
336, 17, 15
(2)
Jutogh “paragneiss”
W50
765
150, 27.4, 12.8
(3)
Jutogh “paragneiss”
W54
805
279, 23.4, 18.1
(3)
W60
796
Jutogh gneiss
234, 32.1, 10.2
(3)
W65
818
Jutogh augen gneiss
272, 35.9, 18.2
(3)
HF49/90
840
Metarhyolite
259, 35, 19
(4)
HF39/90
770
Granite
241, 11, 11
(4)
HF46/90
779
Granite
350, 13, 24
(4)
HF102/90
786
Granite
349, 3, 14
(4)
HF106/90
770
Granite
282, 18, 10
(4)
HF35/92
780
Wangtu granite
298, 25, 16
(4)
63i
665
Jutogh leucogranite
111, 15.3, 12.7
(5)
70i
680
Jutogh leucogranite
84, 26.1, 10.0
(5)
70ii
662
Jutogh leucogranite
77, 25.3, 5.4
(5)
Jutogh “pelite”
63ii
810
163, 30.6, 12.9
(5)
Jutogh “pelite”
71iiiy
812
248, 39.7, 16.4
(5)
Jutogh “pelite”
71ii
809
131, 36.1, 14.9
(5)
MJ-43
780
Coarse-grained gneiss
291, 28
(6)
MJ-44
787
Coarse-grained gneiss
285, 29, 21
(6)
MJ-46
778
Coarse-grained gneiss
, 24, 11
(6)
MJ-114
770
Coarse-grained gneiss
335, 19, 15
(6)
MJ-153
846
Coarse-grained gneiss
246, 18, 17
(6)
MJ-154
841
Coarse-grained gneiss
282, 15, 18
(6)
MJ-155
827
Coarse-grained gneiss
246, 19, 19
(6)
MJ-158
751
Coarse-grained gneiss
225, 17, 13
(6)
MJ-160
766
Coarse-grained gneiss
383, 21, 14
(6)
MJC-66
748
Coarse-grained gneiss
404, 15, 14
(6)
MJC-67
820
Coarse-grained gneiss
383, 21, 17
(6)
MJ-11
750
Fine-grained gneiss
511, 13, 27
(6)
MJ-16
788
Fine-grained gneiss
205, 20, 22
(6)
MJ-21
783
Fine-grained gneiss
436, 26, 20
(6)
MJ-22
785
Fine-grained gneiss
329, 25, 17
(6)
MJ-23
804
Fine-grained gneiss
318, 22, 20
(6)
MJ-26
773
Fine-grained gneiss
, 22, 13
(6)
MJ-40
751
Fine-grained gneiss
321, 21, 17
(6)
MJ-41
754
Fine-grained gneiss
300, 24, 14
(6)
MJ-42
758
Fine-grained gneiss
255, 30, 17
(6)
MJ-149
769
Fine-grained gneiss
288, 27, 11
(6)
MJ-151
748
Fine-grained gneiss
332, 29, 14
(6)
MJ-152
785
Fine-grained gneiss
267, 30, 18
(6)
MJ-156
824
Fine-grained gneiss
644, 144, 21
(6)
MJ-157
763
Fine-grained gneiss
383, 32, 22
(6)
SV798
840
Metatuff
122, 33, 11.5
(7)
AS01-5
759
Ulleri gneiss
319, 30, 17.9
(7)
LT01-44a
788
Metatuff
86, 29, 9.6
(7)
“ Ulleri” gneiss
LT01-102
895
92, 22, 7.8
(7)
AR01-10c
686
Metagranite
615, 19, 19.3
(7)
AR01-15
780
Metagranite
379, 24, 15.9
(7)
AR01-40b
801
Metagranite
N.D.
(7)
Note: Zirconium-saturation temperatures calculated according to Watson and Harrison (1983). Data sources
include: 1—Le Fort and Raï (1999); 2—DiPietro and Isachsen (2001); 3—Richards et al. (2005); 4—Miller et
al. (2000); 5—Chambers et al. (2008); 6—Sharma and Rashid (2001); 7—this study.
Geological Society of America Bulletin, March/April 2010
333
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