Zircon U-Pb and galena Pb isotope evidence for an approximate 1.0 Ga terrane constituting the western margin of the Aravalli-Delhi orogenic belt, northwestern India

Zircon U-Pb ages of 987 +/- 6.4 and 986.3 +/- 2.4 Ma have been established for rhyolites in the southern and northern parts, respectively, of the Ambaji-Sendra are terrane in the western fringe of the Aravalli-Delhi orogenic belt. Pb isotope data for volcanogenic massive sulphide deposits within this are terrane define a linear trend, which is considered a useful reference palaeoisochron or mixing line at an age of about 990 Ma, and establish continuity of the terrane between Ambaji and Sendra. These results are in contrast to the traditional and continuing assignment of the Ambaji-Sendra terrane to the > 1700 Ma Delhi Supergroup. The isotopic composition of galena from an apparently epigenetic occurrence at Punagarh Hill yields a model age of about 940 Ma, suggesting that the Punagarh Group could form part of the same are sequence. The Phulad lineament, which has been considered by most workers to represent the western boundary of the Aravalli-Delhi orogenic belt, appears, in terms of this stratigraphic assignment, to represent an oblique structure? which has not greatly offset the Punagarh and Ambaji-Sendra domains within the are terrane. The eastern boundary of the terrane is marked by the Sabarmati fault. A zircon U-Pb age of 836 + 7/ - 5 Ma for the Siwaya gneissic granite, in the southern part of the Ambaji-Sendra belt, is in accord with previous age data for felsic Erinpura plutons that have intruded the are sequence. A monazite age of 826 +/- 5 Ma may reflect slow cooling of the Siwaya pluton or a younger thermal event. A model age of about 820 Ma for galena is obtained from the Tosham Sn-Cu mineralized zone, in Haryana state, about 280 km north-northeast of Ajmer, which is related to the felsic. anorogenic, Malani volcanism-plutonism. Hence, this widespread magmatism, found extensively west of the Ambaji-Sendra terrane, may have been coeval with Erinpura plutonism or followed it very closely. The present geochronological data warrant the recognition of the Ambaji-Sendra are terrane, as defined here, as a distinct metallogenic province, which saw the formation of VMS-type deposits in the late Mesoproterozoic, around 1.0 Ga. The Pb-isotope compositions of the deposits however, do not clearly define the metal sources. The lead from Danva prospect is most primitive and must have the greatest component from a juvenile mantle source. The Birantiya Khurd deposit, on the other hand, contains a much greater component of lead from a significantly older crustal source. (C) 2001 Elsevier Science B.V. All rights reserved.