Geochemistry And Tectonic Implications Of Gabbros And Granites In The Northern Alxa,NW China

The Alxa area plays an important role in interpreting the Late Paleozoic tectonic evolution of the central part of the Southern Central Asian Orogenic Belt?CAOB?.This paper presents geochronological and geochemical data for Early Permian gabbros and Late Triassic granites from the Yagan area in northern Alxa.1.New LA-ICP-MS U-Pb zircon dating results reveal that the gabbros crystallized at 282.2±3.9Ma?MSWD=2.0?.The gabbros are characterized by low contents in SiO₂?49.89 wt%?,Na₂O?1.94 wt%?,K₂O?0.55 wt%?,TiO₂?0.47 wt%?and P₂O₅?0.09 wt%?,and high contents in MgO?9.1 wt%?,CaO?12.76 wt%?,Al₂O₃?15.67 wt%?and total Fe₂O₃?7.4 wt%?.Its REE pattern???LREE/?HREE?=3.58⁴.32,?La/Yb?_N=3.23⁴.32?is similar to that of E-MORB.It shows depletion of Nb,Ta,Ti,Zr and Hf,and enrichment of Rb,Ba,Th,U and Pb et al.What`s more,the gabbros have?Nd?t??-0.91^-0.54?and?Hf?t??2.59⁶.37?.All these features indicate that the Early Permian gabbros were derived from an depleted mantle metasomatized by subduction fluids,and crustal contamination is involved during the process of magma migration and emplacement.The tectonic setting should be an active continental margin related to the southward subduction of the ocean represented by the Yagan Fault Belt.2.New LA-ICP-MS U-Pb zircon dating results reveal that the granites crystallized at 216.3±3.2Ma?MSWD=2.5?.The Late Triassic granites display high contents in SiO₂?76.88 wt%?,and low contents in total Fe₂O₃?1.03 wt%?,CaO?0.80wt%?,TiO₂?0.13 wt%?and P₂O₅?0.02 wt%?.The contents of total alkalis?Na₂O+K₂O?and Al₂O₃ are 8.46 wt%and 11.78 wt%,respectively,which indicate affinities to high-K calc-alkaline metaluminous to weakly peraluminous granite.Its REE pattern are characterized by strong enrichment in LREE and depletion in HREE.It also shows depletion in Nb,Ta and Ti,and enrichment in Rb,K,Th,U and Pb et al.Furthermore,those granites present positive?Nd?t??1.55¹.99?and?Hf?t??5.03⁷.64?.Those features reveal that the Late Triassic granites were derived from the mixing of mantle and crust and formed in a postcollisional setting.3.The gabbros in the Yagan area in the northern Alxa area formed in an active continental margin tectonic setting.The granites in the region formed in a postcollisional setting.Considering previous studies related to this area,we conclude that the branch of the Paleo-Asian Ocean?PAO?represented by the Yagan Fault had closed in the Late Permian to Early-Middle Triassic and the studied area entered into postcollisional setting in the Late Triassic.