Geochemistry and petrogenesis of Lower Paleozoic meta-granites of Lahaul-Spiti region, Himachal Pradesh, India.

Abstract

In the Lahaul-Spiti region of Himachal Pradesh two meta-granite bodies (Jispa granite and Ratilaman granite) emplaced in the low grade Haimanta Formation which is tectonically overlying the Central Crystallines. Field, petrographical and geochemical features of these meta-granites are compared with the orthogneisses of the Central Crystallines. Geochemical signatures of these meta-granites and orthogneisses suggest that they are products of peraluminous melt generated by partial melting of older crustal rock, with high A/CNK values and higher normative corundum. Decrease in Fe203, MgO, CaO, Ti02, Sr and Zr and enhancement in K20, Si02 and Rb as well as Rb/Sr ratios from melanocratic (margin) to leucocratic (core) portion of these bodies indicate that they are consistent with fractional crystallization processes. Decrease of total REE, moderate LREE/HREE ratios and more pronounced negative Eu anomalies are also in accord with fractional crystallization.The heat source for these lower Paleozoic granite magmas may be due to the lithospheric extension which resulted in mantle melting beneath the crust which led to anomalous heat flow, and triggered crustal melting. Felsic magmas thus produced have intruded into shallow crustal levels. INTRODUCTIONIn the Himalayan region the 500 Ma granitic emplacement is probably the most widespread phenomenon around the Cambro-Ordovician boundary and this event can be followed in some 5000 km distance across Antarctica upto central Australia (Le-Fort et al., 1986). These early Pa leozoic meta-granites are concentrated in the crystalline thrust sheets of Lesser Himalaya, in the Higher Himalayan Crystallines and also in the metamorphosed basal portions of the Tethyan sedimentary sequence. Meta-granite bodies belong to this period have also been reported from the northern Himalaya or Lhagoi-Kangri plutonic belts (Le-Fort et al., 1986).These early Paleozoic meta-granite bodies have common petrographical and geochemical charac teristics and they are peraluminous with high ini tial Sr isotopic ratio (Le-Fort et al., 1986). These early Paleozoic meta-granites in the NW-Himalaya have been deformed and metamorphosed during the Himalayan Orogeny.In this paper two such meta-granite bodies of lower Paleozoic age, namely, the Jispa and Ratilaman occurring in the low grade Haimanta Formation of Lahaul-Spiti region are taken up for their petrogenesis.The geochemistry of these meta-granites is compared with the orthogneisses occurring within the Rohtang gneissic complex (Higher Himalayan Crystallines) for source char acteristics and genetic relationships. GEOLOGICAL SETTINGThe Higher Himalayan Crystalline rocks of the Lahaul-Spiti region is classified into two main tectonic units, namely the lower Central Crystallines or the Rohtang gneissic complex and the upper Haimanta Formation (Fig. 1). These two units are separated by a thrust, designated as Tethyan thrust (Choudhury et al., 1992; Rawat and Thakur, 1988). In the southeast Zanskar and Lahaul region (part of the investigated area), (Fig. 1)...

Alkali-Alumina and MgO-Alumina Molar Ratios of Altered and Unaltered Rhyolites

“…Cenozoic Barnes et al, 2001 Debon et al, 1986Inger and Harris, 1993Thorpe et al, 1990 Paleozoic Azevdo and Nolan, 1998Croudace, 1982 Debon et al, 1986Islam and Gururajan, 1997Jenner et al, 1991Muir et al, 1996Pe-Piper et al, 1991Pressley and Brown, 1999Tate and Clarke, 1997Wiebe and Ulrich, 1997Sun et al, 1993 Proterozoic Annells, 1974Annells, 1973Bourne, 1991Cullers et al, 1993Frost et al, 1999 Goellnicht el al., 1991Jelinek and Dudek, 1993Pollard et al, 1991 Archean Ayres et al, 1991Barley et al, 1984Beakhouse and McNutt, 1991Cassidy et al, 1991Davis et al, 1994 De Ronde et al, 1991 Fend and Kerrich, 1992Gosselin et al, 1990Gower et al, 1983Noble, 1989Paradis et al, 1988Sage et al, 1996bShirey and Hanson, 1986Sun et al, 1993Thurston and Fryer, 1983Witt and Swager, 1989 Volcanic Rocks Cenozoic Baitis and Lindstrom, 1980 Bindeman andValley, 2001 Archean Barley et al, 1984 Fowler and Jensen, 1989Hollings et al, 2000 Jenson and Langford, 1983Muirhead and Hutchinson, 1999Noble, 1989Prior, 1996Thurston and Fryer, 1983Thurston and Fryer, 1983…”