The Xiaohaizi complex in Bachu County,Xinjiang,is a part of the Early Permian Tarim Large Igneous Province in northwestern China.It is composed of mafic-ultramafic intrusions,alkaline intrusions and dykes.In order to better constrain its genesis and the metallogenic conditions of associated Fe-Ti oxides,we conducted a compre-hensive study of mineralogy,geochemistry and zircon U-Pb dating of the first-stage mafic-ultramafic intrusions(gabbro,pyroxenite),the second-stage alkaline intrusions(pyroxene syenite,K-feldspar granite)and the later dikes(diabase dikes,diorite dikes and syenite dikes).Zircon dating results show that the age of gabbro is 286.1～285.8 Ma,the age of pyroxenite is 282.3 Ma,the age of pyroxene syenite is 279.4 Ma,the age of K-feldspar gran-ite is 275.3 Ma,the age of diabase dyke is 284.9 Ma,the age of diorite dyke is 274 Ma,and the age of syenite dyke is(275.5±1.8)Ma,which are consistent with the successive intrusion relationship from geological observa-tion,and indicates that the Xiaohaizi complex(274～286 Ma)was formed in the second stage of magmatic activity in the Tarim Large Igneous Province,and the evolution time is about 12 Ma.The geochemical data show that the Xiaohaizi igneous rock has a composition affinity similar to O1B,similar to the contemporary basalt in the Bachu area.The iron-titanium oxide ore is mainly hosted in pyroxenite and gabbro,which may be formed by deep mag-matic differentiation and subsequent fractional crystallization.The mafic-ultramafic rocks of the Xiaohaizi com-plex have similar trace element patterns to basalts in the Bachu area,indicating that they may have similar mantle sources,which may derive from the subcontinental lithospheric mantle source indicated by subduction-related processes.The trace element patterns of the Xiaohaizi pyroxene syenite and K-feldspar granite are nearly the same,and the Th/U and Zr/Nb ratios are 3.6～4.7 and 4.6～6.0,respectively,suggesting that they may derive from the same mantle source and formed by partial melting or fractional crystallization of mafic rocks or deep-seated magmas with the same parent magma.All kinds of dikes in this area have the trace element geochemical character-istics of basalt formed in intraplate extensional environment.In the late stage of magmatic differentiation,the accu-mulation of Fe-Ti oxides may be accompanied by the crystallization of Fe-Ti melts under high f(O2)and volatile-rich conditions.Most of the Fe-Ti oxides crystallize in the form of irregular aggregates in the late stage.The aggre-gation of disseminated and interstitial magnetite may be caused by the change of Fe2O3/FeO ratio,f(O2)and pres-sure in the liquid phase,or the volatile content of silicate magma in the process of separation and crystallization.
关键词
塔里木大火成岩省/镁铁-超镁铁质岩体/年代学/岩浆演化/Fe-Ti氧化物/新疆
Key words
Tarim Large Igneous Province/mafic-ultramafic intrusion/chronology/magma evolution/Fe-Ti oxide/Xinjiang
维普
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