Imprint of Himalayan Orogeny on Pan African Granitoid Intrusives: Evidence from Dhaoladhar Granite NW Himalaya, India

Introduction

Geology of Himalaya is very complicated due to structural complexity of the rocks and discontinuous nature of most of the lithounits. A typical problem is there with regard to their age and origin. Both, thick and thin skin tectonics, have played important role in tectonic modification of the Himalaya. There are concrete evidences of block faulting and tear faults, too, in Himalayan Foothills (HF) and Lesser Himalayan Belt (LHB). Due to intensive thrusting at various levels, varied lithologies, some as old as Palaeoproterozoic (?), have been exposed and the Tertiary sedimentary prism has been acutely disturbed. These thrusts are getting younger successively due South, indicating varied intense episodes of orogeny.

LHB is structurally demarcated to its south by a regional thrust called the Main Boundary Thrust (MBT). This thrust has surficially delineated a Tertiary siliciclastic sedimentary prism to its south from essential pre-Tertiary lithology consisting of marginal clastic, carbonate, volcanoclastic and marine volcanics (Shali-Sundernagar). Chail thrust brought the allochthonous Chail metamorphics and Dhaoladhar granite over the Pre-Tertiary parautochthonous sequence mentioned above. Various granitoids contained in these thrusts were assigned ages ranging from Proterozoic, Early Palaeozoic to Late Cenozoic by many workers mainly based on field relationship, petrographical simmilarities, degree of metamorphism, etc. Though tectonics of these granitoids are poorly understood, nevertheless two main granitoids and granite gnesis have been distinguished in relation to the Himalyan orogenesis, viz. Pre-collisional granite/gneiss and post-collisional intrusives. Dhaoladhar granite is a linear strip of granitoid body mapped all along the north of Chail right from Dalhausie to the north-east of Jogindernagar in Himachal Pradesh.

Geochronological studies over the past years have contributed significantly in better understanding and delineation of the Himalyan granitoid belts. Age data reveal distinct patterns of magmatic activity in the Himalaya, wherein numerous ages of these bodies distinctly cluster around 1800-2000 Ma, 1200-1400 Ma, 500 Ma and few dates around 1000 Ma. These ages constraints are mostly based on Rb-Sr whole rock isochrons and few U-Pb zircon ages and reveal the presence of Early-Middle Proterozoic basement rocks, emplaced by early Paleozoic graintoids and intense crustal remobilization during the Himalayan orogeny. The post collisional leucogranite intrusion has occurred in four distinct phases : Eocene (50-35 Ma) in the NW Higher Himalaya, Early-Middle Miocene (24-15 Ma) in the Higher Himalaya, Middle to Late Miocene (15-7 MA) in the northern Himalaya in south Tibet and Pliocene (7-2.5 Ma) in the Nanga Parbat massif1.

The Pan-African magmatism is marked by emplacement of series of granite/plutons during the early Paleozoic as distinct linear bodies in the Lesser Himalayan granitic belt (Le Fort et al.,1980, 1983). In order to establish an event stratigraphy, geochemical and radiometric characteristic of the Pan African magmetic activity and the effect of Himalayan orogeny on it, an attempt has been made in this work to understand the origin, evolution and age of Dhaoladhar granite by K-Ar and Rb-Sr method from the allochthon block of the western Himalayan sector.

Geological setting

A continuous strip of granitic mass is found extended all along the strike (ENE-WSW) from Dalhausie to the southwest of Mandi. The topographically high reaches of Dhaoladhar range is marked by this rocks only. Tectonostratigraphically the granitic rocks are lying over the argillometamorphic sequences of Chail. The contact is sharp at places but the effect of migmatisation is common at most of the places.

Though there are a number of orogenic episodes attributed by the earlier workers (Srikantia, 1977; Sinha, 1977) during the Precambrian time, e.g. Shali, Sundernagar orogeny etc., the main structural modification of this area took place during Tertiary time along with Himalayan orogeny. In the southern sector of Lesser Himalaya the successive older lithounits are lying over the younger lithounits , and separated by regional thrust planes. The only exception is the Dhaoladhar granitoid, which is younger to chail, and though is lying structurally above the later is not a typical thrusted unit.

The tectonostratigraphy of the area shows a sequential stacking of different lithounits along thrusts. The southern contact of Dalhausie-Mandi trap is Main Boundary Thrust. The base of Chail is Chail Thrust, which is again marked by a mylonitised augen gneiss. All the lithounits are found in normal sequence i.e. younging due north. A reverse sequence of metamorphic facies is observed in Chail which shows the lowest grade(Chlorite) at the oldest unit and the highest (Garnet) in the youngest, at the contact of Dhaoladhar granite. Srivastava and Singh (1972) invoked the concept of isoclinal overturned fold with the granite at its core is found not justified as the ground data shows no evidence of regional overturning the Chails. Dhaoladhar granite maintains the same trend of foliation with Chails. Xenoliths of Chails having the same structural trend within it refers a syn-kynametic, layer parallel grantisation, not a forceful piercement. Evidences of thrusting viz. mylonitisation, shearing etc. are very common at the contact of granite, hence it is considered as tectonic.

Field relationship and petrography

The contact of Dhaoladhar granitoids and Chail metamorphics is not always very sharp. Effect of migmatisation is very common in the transition zone. Garnetiferous mica schist, at the topmost structural level of Chail shows different degree of migmatisation and sometimes the host rock is found completely transformed to granite gneiss. Thick xenoliths of metasediments, maintaining structural continuity of the Chail metamorphics are present at different levels within the granitoid complex (Bir-Billing road, Bajgar Khad). This indicates partial fusion of the host rock and local generation of anatectic melt along structurally weak planes, which permited the country rock to generate synkinematic gneisses. The high initial 87Sr/86Sr ratio (0.712) also suggests the remobilisation of older sediments to be responsible for the generation of metasomatic granitic gneiss. Srikantia and Sharma (1969) proposed a plutonic origin of this granitoid but all the evidences gathered by the present study suggests its migmatitic origin. Agarwal (1996) also reported that the plutonic nature of this granite is not visible in seismic section.

Most of the samples of the area under study are collected from the southern margin of Dhaoladhar Granitoid. The rocks are generally gneissic. Foliations are marked by flakes of muscovite and biotite, which alternates with quartzofeldspathic materials. The distinct physical varieties are recoded in field.

• Foliated
• Augen and
• Porphyroblastic

The foliated variety is leucocratic, coarsely crystalline, marked by alternate bands of mica (mostly biotite) and light coloured minerals i.e. quartz, feldspar. This type is well developed upstream of Awa Khad and Setu Nala, east of Palampur and is not laterally persistent in other tributaries of this area.

The augen gneiss is leucocratic to mesocratic with subequal proportion of quartz and feldspar along with muscovite and biotite. Degree of crystallinity is variable from fine to coarse grained. Feldspar augens are well developed within light coloured bands with curvilinear foliation of platy minerals around the augens. Feldspar is mostly potassium rich and contains inclusion of biotite within augens. This variety is recorded in lenticular patches and strips, north of Baijnath along Kharas, Sansal and Bir Khads and the only report from the western part of the area is from Lingty Khad, northwest of Palampur.

The porphyritic type is most common among all other varieties of Gneissic rocks along the southern margin of Dhaoladhar Granitoid. Idioblastic feldspar crystals, ranging in size from 4mm to 3cm, are potassium rich, set in a groundmass of leucocratic granite. Biotite, the common mica mineral sometimes occurs as very coarse books. Very large Porphyries are present in Naugal and Banu Khad, two major tributaries north of Palampur and Baijnath respectively. Intermediate size of porphyries is reported from Iku, Buner, Bugh, Bajgar and Gugli Khads. Broad mineralogical composition of porphyritic granite and augen gneiss of Dhaoladhar granitoid is given in Table 3.

Sample details and methidology

Dhaoladhar Granite and augen gneiss samples were collected from the allochthon block from a more or less lateral spacing of 1.5 to 2 km within a belt of 5-10 km stretch from Jogender Nagar to Palampur.

The location map of the studied samples and generalised tectonostratigraphy is shown in Fig. 1 and 2.

Sample Preparation

For isotopic studies, fresh whole rock samples were broken into small pieces and ultrasonically washed with triple distilled water to remove surface contaminations, if any. Sample pieces containing mineral veins and inclusions were discarded and the remaining pieces were then crushed to 2-3 mm size using a jaw crusher. The surface of the pulverizer was cleaned thoroughly with acetone before and after each sample and preconditioned with a small amount of the sample being processed. Crushed samples were further powdered using a Retsch grinder, sieved between 200-300 mesh size and stored in pre-cleaned polythene bags. These fractions, which are homogenous and representative of the whole rock samples, were used for Rb-Sr isotopic studies. Maximum care was taken to prevent any cross-contamination.

For biotite and Feldspar mineral separation, the samples were crushed and sieved in the range of 40-50 mesh size. After obtaining various size fractions, the samples were first thoroughly washed with tap water and then repeatedly with triple distilled water to ensure removal of all dust particles from the mineral fractions. The samples were then dried at around 50° C in an oven.

The clean mineral fractions were then fed into a magnetic barrier separator first at 0.1 amp. current to remove all the high magnetic materials i.e. magnetite and haematite etc. and then at 0.35 amp. current to separate the biotite. The biotite fractions, collected from the magnetic barrier separator, were further purified using gravity separation method in heavy liquid (bromoform) by which lighter fractions containing mixture of biotite and feldspathic material were removed. The denser and lighter portions (purified biotite and feldspar) obtained by this process were repeatedly washed with acetone to remove the bromoform stains. They were then cleaned with triple distilled water for 15 minutes and with acetone for 10 minutes in an ultrasonic bath, to remove adsorbed and unwanted chemicals, if any, on the surface of the minerals, which may affect measurement of their isotopic compositions. The cleaned fractions were then dried at low temperature at around 50°C. Final purification step involved hand picking of the impurities under binocular microscope to get around 99% pure biotite and feldspar fraction.

Rubidium- Strontium Analysis

Two sets of about 150-200 mg each of the whole rock and separated minerals (one unspiked and the other spiked with 84Sr and 87Rb) were weighed on a Mettler AE-240 balance in 25 ml teflon beakers and were moistened with a few drops of high purity triple distilled water. The samples were digested in pressure digestion bombs at 150°C for 12 hr with mixture of acids (4 ml HF+3 ml HNO3+2 ml HClO4). Samples were then evaporated to dryness at 90°C and one more acid treatment (HF+HNO3) was given to ensure complete digestion. After evaporation of the acid mixtures, the residues were dissolved in about 5 ml of 6N HCl and dried before finally making the solution in 3 ml of 2.5N HCl.

Rb and Sr from the digested samples were separated using ion exchange chromatography. The Rb and Sr isotopic analyses were carried out on a VG 354 Thermal Ionisation Mass Spectrometer using peak jumping programme. The data were processed on an on-line computer. In general, 200 and 60 scans were run for measurement of average Sr and Rb isotopic abundances, respectively. The measured 87Sr/86Sr ratios were corrected for mass fractionation by setting 86Sr/88Sr = 0.1194. The mean value of NBS 987 Sr standard during this work was 0.710219±0.000058 (2).

Result and discussions

Granite Geochemistry

Dhaoladhar granite has a distinctive geochemical signature, typical of S-type granites (Willis-Richards and Jackson, 1989). It is silica-rich (67-74% SiO2), rich in K2O with high K2O/Na2O (1.15-2.75) ratios (Table 1). CaO and MgO concentrations are moderate to low. It also has high concentration (relative to most granitic rocks) of Rb, Cr, Ni, U and Th and comparatively low Sr. The normative calculation (Table. 2) of the whole rock samples when plotted on quartz- alkali feldspar-plagioclase (QAP) normative plot of Streikensen (1976) fall in the field of granite and alkali granite (Fig. 3). Harker diagram shows negative correlation for most of the oxides against SiO2 (Fig. 4), possibly indicating an early crystallisation of alkali-feldspar, biotite and other accessory mineral phases. Rb/Sr (1.2-6.5) and Ba/Sr (2.1-8.0) ratios are high. Rb - Y+Nb and Nb - Y discrimination diagram (Pearce et al., 1984) reveal that the samples plot in Syn-COLG (syn-collisional granite) (Fig. 5 a & b). The high initial 87Sr/86Sr ratio (0.712), relatively low sodium content, and presence of ilmenite, hematite, apatite, garnet and muscovite are diagnostic of S-type granites.

They are basically peraluminous rocks, biotite rich with common oxides as ilmenite. The geochemical nature suggests that they are produced by partial melting of already peraluminous sedimentary source rock. Winter (2001) has shown that melting of the sialic crustal rocks produces syn-orogenic S-type granitoids. LeFort (1988) has shown that there are five separate belts of granitoid intrusions parallel to the axis of the Himalayas, each with its own compositional, temporal and genetic relationships. France-Lanord and Le Fort (1988) proposed that melting of the gneisses was induced by the introduction of hydrous upper crustal rocks of the Indian plate that were pushed beneath the Main Boundary Thrust.

One very important and interesting observation about the Dhaoladhar granite is its linear shape, having an extremely high length-width ratio. This is a typical sheet like granitoid body. It is compositionally heterogeneous and shows compositional layering commonly concordant to a tectonic foliation. Lucas & St-Onge (1995) consider many of the granitic bodies, comprising alternations of syn-kinematic, layer-parallel, granitic veins and sheets. Pawley, Collins and Kranendonk (2001) proposed on the basis of examples from the Late Archaean granitoids of Pilbara Craton, Western Australia, that many such granitoids form in ductile shear zones during non-coaxial, compressive deformation. Though both the south and northern contacts of Dhaoladhar granite are with Chail Metamorphics, both of the contacts are found tectonic. The structural position of the porphyritic augen gneiss is typically associated with the structurally weak zones, which have acted as conduits for the granitic melts generated at the deeper level of the crust.

Harris et.al.(1986) described a sequence of four series of magmas that are found in many collision zones. The first is a typical pre-collision I-type continental arc series. This is followed by a syn-collisio peraluminous leucocratic series such as the High Himalayan granitoids. Next is a late- or post-collision calc-alkaline series that may be mantle derived, but undergoes considerable crustal contamination. A final series is post collision alkaline intrusives with intraplate signature (lacking high LIL/HFS). A few high Himalayan granites dated between 20-28 Ma. and are synchronous to , may be considered depleted from the same source, and migrated differentially along different thrust planes.

Rb-Sr age of Dhaoladhar Granite

The data on Rb and Sr concentrations and their ratios in Dhaoladhar granite are presented in Table 3.The Rb and Sr concentration of the samples vary from 188 to 334 ppm and 49 to 139 ppm respectively. The 87Rb/86Sr ratio (atomic) of the samples vary from 3.9 to 19.7 and the 87Sr/86Sr ratio from 0.74 to 0.85. The data are regressed as per the scheme of Provost (1990). All the errors are quoted at 2 level and the errors on 87Rb/86Sr ratios are taken at 2%. All the samples yield an isochron age of 511.4±9.8 Ma with an initial Sr ratio of 0.71175±0.00094 (Fig 6). The MSWD of the regressed data is 0.61, thereby, suggesting closer fitting of the data points. This exercise, thus suggests Late Proterozoic/ early Paleozoic metamorphic resetting. The metamorphic resetting is also confirmed by its high initial Sr ratio.

The isotopic ages in the range of 500-550 Ma have been reported by various workers from the Indian Himalayas (Jager et al., 1971; Mehta, 1977; Frank et al., 1977; Bhanot et al., 1979; Singh et al., 1985; Trivedi et al., 1984, Pognant et al., 1990 ; Hohendorf et al., 1991; Kwatra, 1986; Singh et al.,1991; Sarkar et al.,1996; Kishore et al., 1996; Kwatra et al., 1999). Comparison of present study vis-a-vis other workers is shown in Table 4. Paleozoic ages are known not only from the Uttar Pradesh and Nepal Himalayas but also from the root zone in these sectors. The conceivable Peninsular equivalents of these early Palaeozoic granite are likely to occur on a southwestern prolongation into the Peninsula west of Aravalli Range. Rathore et al. (1996,1999) had reported the existence of secondary thermal event around 500-550 Ma in Malani volcanics and Jalore granites from western Rajasthan which are close to Himalayan foothills. Since most of these rocks show secondary event at about 550 Ma (Choudhary et al., 1982), it is likely that the early Paleozoic granites of Himalaya represent late intruded parts in the trans-Aravali terrain. Mehta (1977) relates this time, on the other hand, to the formation of a protoform of the Great Himalayan "Central Crystalline Axis" which was subsequently rejuvenated during the Hercynian and the Himalayan orogenies. Islam et al. (1999) have considered this event (500±50 Ma) to be the result of a strong diastrophic orogenic event correlatable to the late phases of the Pan-African orogeny in Africa.

Rb-Sr age of mineral separates from Dhaoladhar Granite

The data on Rb and Sr concentrations and their ratios in Kharas-15 and Bajgar-45 whole rocks from Dhaoladhar granite and their minerals separate are presented in Table 5 and 6. When the data of the minerals and whole rock of Kharas-15 are regressed it yielded an Rb-Sr age of 26.3±1.1 Ma with initial strontium ratio of 0.73906±0.00021 (Fig. 7). The MSWD of the regressed data is 4.65. Similarly, when the data of the minerals and whole rock of Bajgar-45 are regressed it yielded an Rb-Sr age of 26.42±0.98 Ma with initial strontium ratio of 0.77011±0.00013 (Fig. 8). The MSWD of the regressed data is 3.31.

Geochronological ages in the range of 20-30 Ma, mainly by U-Pb, have been reported by various workers from the Indian Himalayas (Hamet & Allegre, 1978; Scharer, 1984; Scharer et al., 1986; Deniel et al., 1987; Copeland et al., 1988; Stern et al., 1989; Parrish & Tirul 1989; Copeland et al., 1990; Hodges et al., !992; Parrish & Hodges, 1992; Harrison & McKeegan., 1994; Harrison et al., 1997; Searle et al., 1999; Nobel & Searle, 1995; Walker et al, 1999; Schneider et al., 1999) (Table 7).

Jagger et al., (1971) has reported apparent biotite ages from the Mandi granite in the range of 24-31 Ma pointing towards the intense phase of Himalayan metamorphism during that period. Rb-Sr ages from the Zanskar shear zone of North West India which forms the western segment of the south Tibetian detachment system show that ductile deformation was ongoing at 26 Ma (Inger, 1998). Dating of the Manaslu granite by conventional U-Pb method on zircon and monazite has yielded ages of 24 Ma and 21.9 Ma, respectively (Scharer, Xu and Allegre (1986). Hodges et al. (1996) showed that this zone had been active from atleast 22 Ma. The ages obtained by Rb-Sr technique are consistent with published 40Ar-39Ar data of Searle et al. (1992) that suggests cooling below 500 C before 30 Ma for the Sankoo granite and around 20 Ma for deeper High Himalayan crstalline rocks.

The 25 Ma isochron age of Dhaoladhar whole rock (Kharas-15 and Bajgar-45) and their mineral separates, can be interpreted as the time elapsed since the last tectonothermal event in these rocks which resulted in resetting of the biotite mineral under green schist facies metamorphisn. By analogy, it can be stated that the biotite in the Dhaoladhar Granite has remained below 300 C temperature (the Rb/Sr closure temperature in biotite), since 25 Ma ago.

Conclusions

In the conclusion, it appears that there are dozens of acidic igneous plutons related to the Pre-Himalayan orogeny introdued during Early Palaeozoic time [~ 500 Ma] in the different tectonic setting all along the Himalayan Range. Dhaoladhar granite is an example of such palaeosubduction event, at the northern margin of Indian subcontinent. Remobilisation and subquent anatexis of supracrustal metasements including the preexisting intrusives took place during the late Oligocene to Early Miocene time, that can be linked with shear heating on a continuously active decollement at the deeper part of the subduction prism. The Higher Himalayan granites are well correlatable with this thermal activity. The in-sequence trusts with their active shear zone played an important role of conduit for the mobilisation of anatectic fluids at shallower levels of the crust. Besides MCT, other deep rooted basement controlled thrusts generated at the level of Chail, Jutogh and their equivalents also played an important role for such fluid migration.

Acknowledgments

The authors wish to express their deep sense of gratitude to Dr Debabrata Ray, GGM & Head, KDMIPE for taking keen interest in this work and for his constant encouragement. Help and inspiration received from Sri K. N. Misra, GM (GRG) and Mrs. N. J. Thomas, DGM (Chem.) are gratefully acknowledged. Motivation and help received from Dr. Sandeep Singh, Department of Earth Sciences, IIT, Roorkee is also thankfully acknowledged. The authors also indebted to T Chatarjee, Dy. Suptdg. Geologist and M. Shukla, Dy. Suptdg. Geologist, from Geological field party for continuous help and for providing maps and geological settings of the area.

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