Early Cretaceous Bimodal Magmatism In The Eastern Tethvan Himalayas,Ibet:records On Precursory Continental Rifting And Initial Breakup Of Eastern Gondwana

We investigate a bimodal magmatic suite of gabbroic-granitic rocks in the Cuona area of the eastern Tethyan Himalayas.Whole-rock major and trace elements,SHRIMP zircon U-Pb geochronology and Hf isotopic analysis were conducted to reveal the petrogenesis of the bimodal suite and through this to explore the tectonic implications.Our main findings include the following.First,the gabbros and granites were emplaced synchronously at 142-135 Ma,considering the field relationships they comprise a bimodal suite.Second,the gabbros are enriched in light rare-earth-elements(LREE),depleted in heavy rare-earth-elements(HREE)with slight Eu anomalies,and have ?Hf(t)values of+0.04-+8.30,which as a whole,suggests that they were derived from an ocean island basalt(OIB)-like mantle source.Third,the granites display low MgO,CaO,and Al2O3,marked enrichment in LREE,highly refracted HREE with strongly negative Eu anomalies and sHf(t)values of-2.01 to+1.71,all of which ultimately gives the characteristics of Al-type granites.Based on these observations,we propose that the bimodal suite is formed in a continental rift.Specifically,the gabbros originated from the Kerguelen plume,whereas the granites derived from the upper crust with garnet-bearing metapelite.The formation mechanism of the bimodal magmatic suites are likely to be consistent with the mixing model.We suppose that the bimodal magmatism represents a rifting event between Greater India and western Australia in eastern Gondwana.This rifting event initiated at 140 Ma,which places bounds on the initial breakup of eastern Gondwana,specifically the separation of Greater India from western Australia.