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Description
The Indian Hill pluton is one of several peraluminous, garnetiferous plutons that occur in the eastern zone of the Peninsular Ranges batholith. It intruded a sillimanite-bearing migmatitic gneiss complex containing isolated bodies of quartzite, marble and amphibolite. The pluton consists of two facies: a medium-grained, garnet-muscovite-biotite granodiorite; and a younger, less abundant, fine-grained, muscovite-biotite grano-diorite. Pegmatitic pods are not developed in either facies. The emplacement of the pluton was preceded by the injection of phaneritic to pegmatitic leucocratic granodiorite dikes. The dikes are garnetiferous and of similar composition to the Indian Hill pluton. They occur exclusively within the metamorphic complex and are truncated by the plutonic units. Major element analyses indicate a limited Si02 range (73%-76%) and A/CNK values from 1.14 to 1.20. Trace element compositions suggest a possible mixing relationship between the Indian Hill and La Posta magmas. The age difference, however, between the two plutons (89 Ma for Indian Hill and 94 Ma for La Posta) does not support mixing unless the cited La Posta age is not representative of that unit in the study area. Garnet-plagioclase-biotite-muscovite thermal and barometric calculations for the garnetiferous facies yield emplacement conditions of approximately 7 00°C and 2.5 ± 0.5 kb (7 to 10 km), respectively. Whole-rock oxygen isotopic ratios vary from 11.7 to 12.1 per mil with the quartz separates ranging from 12.6 to 13.2 per mil. Rubidium/strontium analyses of whole-rock and various mineral separates yield an initial 87Sr/ 86Sr ratio of 0.70628 ± 0.00025 and a regression age of 90 Ma ± 3 Ma. Zircon Pb/U analyses give a lower intercept age of 85 Ma ± 6 Ma and an upper intercept age of 1162 Ma ± 43 0 Ma. Based on the narrower range of the Rb/Sr age, the preferred emplacement age for the Indian Hill pluton is 89 Ma. The data indicate an origin by fractional melting and emplacement of three successive melts which underwent little in situ fractionation. The source region for the Indian Hill pluton consisted of a mixture of oceanic and continental crust. The data also infer that the continental source rocks either contained detrital zircon derived from a Precambrian source or consisted of a combination of Precambrian and Paleozoic rocks. The most likely heat source for fractional melting seems to be the large La Posta pluton which surrounds the metamorphic
