Palaeozoic granites and their younger components - A study of Mandi and Rakcham granites from the Himachal Himalaya

HIMALAYAN JOURNAL OF SCIENCES | VOL. 5 | ISSUE 7 (SPECIAL ISSUE) | 2008 82 Several occurrences of Palaeozoic granites are recorded from the Lesser Himalayas as well as from the Higher Himalayas (Miller et al. 2001). From Himachal many such bodies have been dated (Bhanot et al. 1979, Frank et al. 1977, Jager et al. 1971, Kwatra et al. 1986, Pognante et al. 1990, Kwatra et al. 1999, Kundu et al. 2006). All of these bodies are deformed and several occur in the vicinity of Main Central Thrust (MCT). Studies have indicated presence of more than one granite type in most of these occurrences (e.g. Gupta 1974, Chatterjee 1976) but definitive reference to the Himalayan orogeny has generally been lacking. The present work supplements the earlier field and petrographic classification with rigorous mineralogical including rare earth element (REE) bearing mineral data for the two Paleozoic granites of Mandi and Rakcham and show association of younger granites with these occurrences. Four petrographic variants of Mandi granites can be identified (Chatterjee 1976). These are as follows. 1. Porphyritic granite: With two mica and two feldspar. The ratio of the micas number to feldspar phenocryst vary. The other mineral phases are quartz, ilmenite, sphene, epidote, zircon, secondary muscovite, chlorite, monazite, allanite, zircon, apatite and fluorite. 2. Fine grained porphyritic granite: Two mica two feldspar granite and mineralogy similar to the porphyritic granite but has distinctly finer ground mass size and less phenocrysts. 3. Trondjhemite/albite granite: Leucocratic rock with one feldspar (albite) and one mica (muscovite). Other minerals are quartz, rare biotite, chlorite, wolframite, iron oxides, monazite, fluorite, apatite and tourmaline. 4. Leucogranite/tourmaline granite: Two feldspar and one mica (muscovite) granite, some outcrops have significant amount of tourmaline (more than 1%). Quartz, k-feldspar, albite, tourmaline, muscovite, fluorite, monazite etc. In Rakcham occurrence three variants are identifiable namely 1. porpyritic granite: two feldspar biotite granite, 2. granodiorite and 3. trondjhemite. The latter two variants are subordinate and the major constituent is the porphyritic granite. Magma mingling is indicated by the presence of mafic pillows in the Mandi occurrence (Miller et al. 2001). Mandi Occurrence: In porphyritic granite plagioclase has a range of composition. The larger grains of plagioclase are zoned with core composition (An29) being more calcic than the rim (An23). Finer grained matrix grains are less calcic (An12) and inclusion of plagioclase within K-feldspar is nearly pure albite. Biotite has a high Fe content (Fe/Fe+Mg~ 0.73). The fine grained porphyritic granite has nearly albitic plagioclase and the biotite is more Fe rich (Fe/Fe+Mg~ 0.780). In trondjhemitic granite the Palaeozoic granites and their younger components A study of Mandi and Rakcham granites from the Himachal Himalaya