柴北缘宗务隆构造带蓄集地区闪长岩的发现及其地质意义

Discovery and geological significance of Xuji diorite in Zongwulong tectonic belt on the northern margin of Qaidam Basin

宗务隆构造带是柴达木北缘一个重要构造单元,但其构造属性长期存在争议。本次在宗务隆构造带中西部蓄集地区发现了闪长岩体,对其开展了岩相学、地球化学、锆石U-Pb年代学及Lu-Hf同位素等研究。结果表明,岩体侵入时代为258±1 Ma,富集轻稀土元素而亏损重稀土元素,具弱的负Eu异常,相对富集Rb、K、Sr等大离子亲石元素,而强烈亏损P、Ti、Nb等高场强元素,Cr和Ni含量分别为38.0×10-6~92.8×10-6和6.7×10-6~14.1×10-6,Mg#值为50~56,Rb/Sr值为0.32~0.34,Ti/Zr值为23.26~32.30,Ti/Y值为145.29~163.97,Nb/Ta值为10.45~11.45,Th/La值为0.39~0.51,Ba/La值为15.04~16.60,207Pb/204Pb(i)值为15.602~15.611,锆石εHf(t)值为-8.52~-3.00,二阶段模式年龄值(tDM2)值存在1563~1478 Ma[εHf(t)=-4.37~-3.00]和1827~1790 Ma[εHf(t)=-8.52~-7.98]两组,指示蓄集闪长岩岩浆是壳幔混合的产物,其中古老地壳物质占主导。结合区域地质调查结果,推测宗务隆构造带中西部地区的边缘裂陷应晚于东部。

Zongwulong tectonic belt is an important tectonic unit on the northern margin of Qaidam Basin,but its tectonic properties remain controversial.In this study,a diorite intrusion was recognized in central Xuji area of the Zongwulong tectonic belt,and its petrography,geochemistry,zircon U-Pb chronology and Lu-Hf isotopes were studied.The intrusion time of the diorite is 258±1 Ma;light rare-earth elements are enriched while heavy rareearth elements are depleted,with weak negative Eu anomaly;it is relatively enriched in large ionic lithophile elements such as Rb,K and Sr,but strongly depleted in high field strength elements such as P,Ti and Nb.The content of Cr and Ni are 38.0×10-6~92.8×10-6 and 6.7×10-6~14.1×10-6 respectively,Mg#values are 50~56,Rb/Sr ratios are 0.32~0.34,Ti/Zr ratios are 23.26~32.30,Ti/Y ratios are 145.29~163.97,Nb/Ta ratios are 10.45~11.45,Th/La ratios are 0.39~0.51,Ba/La ratios are 15.04~16.60,207Pb/204Pb(i)ratios are15.602~15.611,zirconεHf(t)values are-8.52~-3.00,and the two-stage Hf model ages are 1563~1478 Ma[εHf(t)=-4.37~-3.00]and 1827~1790 Ma[εHf(t)=-8.52~-7.98].It can thus be concluded that the magma was derived from mixture of the crust and the mantle,where the ancient crustal material was dominant.Combined with regional geological studies,it is suggested that the marginal rifting in the central and western regions of the Zongwulong tectonic belt should be later than that in the east.