|
Holly J. Stein, Colorado State University (United States)
Gang Yang, Colorado State University (United States)
Judith L Hannah, Colorado State University (United States)
Aaron Zimmerman, Colorado State University (United States)
Manoj K Pandit, University of Rajasthan (India)
P.K. Raut, Geological Survey of India (India)
Carmen Gaina, Geological Survey of Norway (Norway)
Trond Torsvik, Geological Survey of Norway (Norway)
|
|
Re-Os dating of ore deposits provides an untapped means for calculating the assembly and dispersal of terranes and their drift over deep plumes and hot spots. Here we demonstrate that Re-Os dating of arsenopyrite associated with small Au deposits on the southern margin of the 1600-km-long Central Indian Tectonic Zone (CITZ) elucidates tectonic processes. The CITZ is a transcontinental Archean-Proterozoic seam binding the southern and northern Indian cratons, collectively constituting peninsular India. The northern margin of the CITZ is seismically active today. The southern margin of the CITZ is marked by the Central Indian Shear Zone (CIS).
Field exposures at the Pular and Parsori Au deposits in the Sakoli belt along the southern margin of the CITZ in central India show that mineralization is hosted in a regional shear zone, close to and parallel to the CIS. Fresh arsenopyrite samples from quartz veins are primarily associated with the contact between Archean metarhyolite tuff and mica schist. The veins are multiply deformed and locally brecciated, with sulfides occurring as disseminations and in en echelon gashes at the hand specimen and larger scale. Re-Os analyses of seven arsenopyrites provide LLHR (low-level highly radiogenic) isotopic compositions. Their high 187Re/188Os ratios (1000), permit single mineral age determinations, as the model age is insensitive to the assumed initial 187Os/188Os ratio. Using an initial 187Os/188Os of 0.2, model ages fall into two groups, 170-150 Ma (n = 4) and 125-100 Ma (n = 3). Although the Pular-Parsori Au mineralization has long been presumed to be Precambrian, we found no evidence for this assumption.
Our Re-Os study presents the first evidence for Mesozoic magmatic-hydrothermal activity in the CITZ. We suggest that the older Re-Os arsenopyrite ages reflect stretching of subcontinental India as it separated from SE Pangea and trekked southeastward toward East Antarctica. This is supported by a precise, 7-point ∼160 Ma Re-Os arsenopyrite isochron with a low and moderately well-constrained initial 187Os/188Os ratio of ∼0.13. The low initial Os ratio does not permit derivation of the Au-bearing fluid from local Archean crust, but requires access to a primitive source in Mesozoic time. After separation from East Africa, India's journey ran south and then north over the Crozet and Marion hot spots. The younger group of arsenopyrite ages is similar to the ∼117 Ma Rajmahal basalt traps in east India, attributed to magmatism associated with the Kerguelen hot spot. Altogether, the Re-Os clock suggests a 50 m.y. separation and rotation history before India commenced its speedy northward trajectory to join Eurasia.
|
