Geochemistry and geodynamic setting of Paleoproterozoic granites of Lesser Garhwal Himalaya, India

The granite and gneisses rocks are well exposed around Toneta, Tilwara and Chirbatiyakhal region in the Lesser Garhwal Himalaya have less studied which consider as Paleoproterozoic age. The granites from Toneta area are classified as K-rich peraluminous granite with low Na 2 O varies from 0.74 to 2.4 wt.% and high K 2 O content varies from 5.0 to 6.91 wt.%. The average Al 2 O 3 (12.7 wt.%) in the granite is greater than the total alkalies (Na 2 O+K 2 O = Av. 7.62 wt.%), the TiO 2 content is low ranging from 0.1 to 0.28 wt. %. In the Y + Nb Rb, Y Nb, Ta + Yb Rb, and Yb Ta discrimination diagram of Pearce et al. (1984) show that the Toneta granites mostly plots within the syn-collision granite fields. This is typical collisional granite.


Introduction
To Granites have provided a constant focus for controversy among geologists on account of their inherent diversity and their association with very wide spectrum of geological phenomenon since the beginning of The Himalayan mountain belt has evolved due to the collision of the Eurasian and Indian Plates and extends along an arc having a convexity towards south (Patriat and Achache, 1984). It measures for about 2400 km in length with a width of about 320 km. The Himalaya is broadly classified as i) Sub-Himalaya, ii) Lesser Himalaya, iii) Higher Himalaya, iv) Tethys Himalaya, and v) Trans Himalaya (Fig. 1a). Lesser Himalayan sequence is separated from the Higher Himalayan crystalline by a deep seated tectonic lineament called Main Central Thrust (MCT; Heim and Gansser, 1939). Fuchs and Sinha (1978), Sinha (1989), Thakur (1992), Gansser (1993) and Saklani (1993) suggest that the MCT is not a single thrust in the Garhwal region, but composed of three tectonic planes i.e. MCT-I, -II and III developed by the duplex mechanism of thrust tectonics (Fig. 1b;Saklani et al., 1991). The extensive work of Valdiya (1980) from Kumaon Himalaya show that several Paleoproterozoic granites bodies occur in Lesser Himalayan sequence and it found all along the 2000 km Himalayan belt starting from Besham in Swat valley (NW Himalaya) to the Bomdila gneiss in the NE Himalaya (Islam et al., 2005;Phukon et al., 2018 and references therein). The granites occurring in the Himalaya are classified in to four groups (Islam et al., 2005): i) Proterozoic granites (2200 -1800 Ma; 1400 -1200 Ma; from Lesser and Higher Himalaya), ii) Early Paleozoic granites (600 -500 Ma), iii) intrusive phases of Ladakh plutonic complex (102 ± 3 Ma and 42 -30 Ma), and iv) Tertiary leucogranites (30 -12 Ma).
Granitic gneisses and granitic augen gneisses exposed in the Lesser Himalayan zone have extended as Proterozoic ages (2200 -1800 Ma; Table 1). These rocks from Lesser and Higher Himalaya are considering as part of the peninsular India, and the occurrences of granites from Lesser Himalayan sequence are well exposed in Himachal and Garhwal -Kumaon regions (Saklani et al., 1991;Saklani, 1993;Singh et al., 1998). The Outer Himalaya and Indo-Gangetic plain formed, after eroded material from Higher Himalaya during Himalayan orogeny, towards south of Lesser Himalaya.  Gansser (1964), and (b) General tectonic map of Garhwal Himalaya after Valdiya (1980). The Lesser Himalayan zone comprises Paleoproterozoic granite, Paleozoic sedimentaries (e.g. slates, phyllites, quartzites) with pene-contemporaneous mafic volcanic (1.83 -1.88 Ga Sm-Nd T-CHUR modal ages; Miller et al., 2000). There is a period of felsic volcanism and granite emplacement occurred around 1.86 1.84 Ga in NW Himalaya. Nd model ages for these granitic rocks extend to 2.63 Ga, indicating recycling of Archean continental crust (Miller et al., 2000). Still more studies are required especially from Garhwal Himalaya where limited data are available to set up Proterozoic magmatism. Therefore, Palaeoproterozoic granites are selected from Toneta area, Tilwara section for whole rock geochemical analysis to establish tectonic environment and geodynamic setting.

Geology of the area
It is supposed that the Indian crust was imbricated and different parautochthonous of deeper rocks from various tectonics levels by the duplex mechanism and thrusted on metasedimentary rocks of the Lesser Himalayan sequence. The study area is constituted by granite, quartzite, schist, gneisses and metabasics (Fig. 2). The undeformed granite exposed around Toneta area is considered as Bhatwari (= Chail/ Ramgarh) group (Valdiya 1980;Saklani et al., 1991). This rock unit has sheared up to 500 meters having brittle ductile deformation (mainly Indian plates basement rocks medium to high metamorphic grade) consider as characteristics of MCT-III zone in the Lesser Himalayan sequence. The rocks of Chail group are tectonically separated by Chail thrust (MCT-III) with massive white quartzite of the Garhwal group towards south (Valdiya, 1980;Saklani et al., 1991;Singh et al., 1998).  Valdiya, 1980). Analysed granite samples are given in Table 2.
The Toneta granites extend from 1.5 km west of Tilwara to Chirbatiyakhal in the east. They are well exposed around Toneta village and hence the name (Negi et al., 1980). These granitic rocks sometime associated with tourmaline granite, gneiss and quartz -sericite -schist (Fig. 3a). Tourmaline granite is medium to coarse grained with feldspar and quartz having subordinate tourmaline and biotite. The sample N-4, N-5 and N-6 are medium to coarse grained containing feldspar (microcline and plagioclase) and quartz, and accessory minerals are biotite and tourmaline. It is typical for MPG (Muscovite-bearing Peraluminous Granitoids) which common for collisional setting (Barbarin, 1999). Striation in tourmaline grains is seen near Toneta, where the grains are 0.5 cm × 0.3 cm (avg.) in dimensions. Near Jakhal village and about 1 km west of Toneta, regular black band of tourmaline are seen in the granites, with a thickness of 0.5 cm to 1 cm and rock exposures occurring around 3 to 7 m bands. There is also sericitization of feldspar. At places the granite has become vary coarse grained, non-foliated porphyritic and rich in biotite (Fig. 3b). N-5 sample is fresh and fine grained rocks occur in very small area about 3km west of Toneta village, exist in a road cutting section and have undeformed (Fig. 3b). The sample N-5 contains quartz and feldspar and subordinate mica minerals with fine grains. The other granite samples are also fresh which have developed crude foliation and schistosity (Fig. 3c).
The granite-gneiss (sample No. S-1, S-4 and S-5) are exposed around Mayali having quartz and flaky minerals. About 1 km south east of Mayali, augen gneisses are observed and some boudinage of quartzites are exposed in the granite gneiss. Similar structures are also reported by Negi et al. (1980) along the Helang gad fault and the rocks are highly scattered. The fine to medium grained quartz-talc-schist is reported by Negi et al. (1980) from Toneta to Mayali traverse which extending further towards North and have elongated quartz grains. Quartz -sericite schist is well developed in sheared zones, particularly along the Alaknanda thrust and in the upper Lastar gad valley and correlated with MCT-III of Saklani et al. (1991) which show thrusted contact with quartzite at southern side. These granites have well developed joint system and also traversed by epidiorites.

Sampling and analytical techniques
Based on criteria least alteration, seven representative samples of granite were analysed. The chemical composition of the rocks was determined at the Testing and Matter Analysis Centre, Institute of Geology, KarRC, RAS, Petrozavodsk, Russia. The detail methodology and precision is described by Svetov et al. (2015), Singh and Slabunov (2015), which profusely discussed in Slabunov and Singh (2018). To estimate rock-forming element concentrations, sample powder was first melted with sodium carbonate, and the resultant melt was then leached with diluted hydrochloric acid. SiO 2 was first precipitated with gelatin from the resultant solution, and its percentage was estimated by the gravimetric method. Al, Fe, Ca and Mg concentrations in the solution were then calculated complexometrically and Ti and P concentrations photometrically. Calculation accuracy was determined from acceptable discrepancy between the results of two parallel estimations which does not exceed 0.7% for Si, 0.5% for Al, Fe, Ca and Mg and 0.3% for Ti and P.
The concentrations of minor elements in the samples were estimated by the inductively coupled plasma mass spectrometry (ICP-MS) method on an Х Series-2 ICP-MS (Thermo Scientific, USA). The samples were decomposed by acid dissolution in an open system. Weighted portions of samples with a mass of 0.1 g were used for analysis. The samples analyzed were decomposed together with reference samples (blank samples) and one standard sample. The major elements and their calculated CIPW weight norms and also trace elements data are given in Table 2 and Table 3.

Geochemistry and tectonic settings
The K-rich granites from Toneta area are mostly calc-alkaline in nature classified as granite (Fig. 4a). The normative feldspar classification diagrams [Albite (Ab) Orthoclase (Or) Barker, 1979] indicate that the Toneta granite falls in granite field (Fig. 4b). In granitoids Na 2 O is low and varies from 0.74 to 2.4 wt.% and K 2 O content varies from 5.0 to 6.91 wt.%. The Al 2 O 3 content varies from 11.67 to 13.7 wt.% except sample N-9 (95% silica). The rocks are highly rich in SiO 2 having range from 71.84 to 76.68 wt.%. Alumina content (Al 2 O 3 : Av. 12.7 wt.% in the granitoids is greater than the total alkalies (Na 2 O+K 2 O: Av 7.62 wt. %), the TiO 2 content in the granitoids are low ranging from 0.1 to 0.28 wt.%. CIPW norms calculations, all the samples show normative corundum values ranging from 0.37 to 3.89 which is a typical in all granite samples of the Lesser Garhwal Himalaya with high normative Orthoclase and less normative Albite-Anorthite feldspar minerals ( Table 2). The granites belong to high-K calc alkaline and shoshonite series (Fig. 5a), with mostly peraluminous in nature (Fig. 5b). Chondrite normalized rare earth-element patterns (McDonough & Sun 1995) of the granitoids show pronounced negative Eu anomaly (Eu/Eu* = 0.18 to 0.36, Fig. 6a), pronounced enrichment in LREE and highly fractionation of heavy rare earth elements (HREE; LaN/LuN = (5.81 to 101.34). These rocks are also rich in LILE: Cs, Rb, Th, U, and Pb (see primitive mantle normalized, McDonough & Sun 1995, Fig. 6b) and display negative Nb Ti anomalies (Fig. 6b).
In the Yb + Ta versus Rb and Nb versus Y discrimination diagram of Pearce et al. (1984), the Toneta granites mostly plots within the syn-collision granite fields (Fig. 7). The R 1 -R 2 [R 1 = 4Si 4+ -11(Na + +K + ) 2(Fe 3+ +Ti 4+ ), molar; R 2 = 6Ca 2+ +2Mg 2+ +Al 3+ , molar] tectonic discrimination diagram show syn-collision to post-orogeny field tectonic setting (Fig. 8). 5. Discussion Phukon et al. (2018) discussed that the granitoids from the Chiplakot Crystalline Belt, Kumaun Himalaya are associated with the Columbia supercontinent subduction system in the Paleoproterozoic. Frank et al. (1977) assign a whole rock Rb-Sr age of 1840 + 70 Ma for Bandal and Sainj granites, documenting an existence of Proterozoic granitic magmatism in the Himalaya. Islam et al. (2005) stated that the Proterozoic granites of the Lesser Himalaya are silica and K 2 O enriched with high A/CNK value (>1.1) and show presence of normative corundum. Islam et al. (2005) also pointed out that the granite, granitic gneisses and associated metasedimentaries rocks in NW Himalaya experienced up to upper amphibolites facies metamorphism which might suggest that the Proterozoic granitic gneisses were derived as basement slivers from middle crustal level (a probable extension of the northern Indian craton). Singh et al. (2009) also indicated that the Paleoproterozoic (ca. 1860 Ma) felsic magmatic rocks are widespread in the Lesser Himalayan Zone to basal part of the Higher Himalayan Crystalline (HHC) probably remobilization of older crustal material which exhumed due to collisional tectonics during the Himalayan orogeny.

Conclusions
To The collision tectonics nature of granites exists in Garhwal Himalaya demonstrate felsic magmatic phase implication occur during Paleoproterozoic ages.
The Lesser Garhwal Himalaya Toneta granites formed syn-collision to post-orogeny tectonic environment, which resemble with ca. 1800-1900 Ma existing hydrothermal activity in Bundelkhand Craton suggest a possible linkage to it.
However, the questions are still unanswered and required more studies to understand about the nature of granitic rocks of Garhwal Himalaya whether these rocks have formed due to the melting and recrystallization of pre existing Archean continental crust of Bundelkhand or Aravalli Cratons?