南秦岭大堡组奥陶纪洋岛玄武岩的识别及其构造意义：来自地球化学和年代学证据

秦岭造山带以商-丹构造带为界分为北秦岭和南秦岭构造带.大堡组位于南秦岭构造带内,由灰色-深灰色的泥岩、粉砂岩和炭硅板岩,以及夹含炭硅质岩、灰岩和基性火山岩块体组成.在大堡组中出露的能干上和蚂蝗峡基性岩块夹于黑色的泥岩之中,岩石地球化学分析结果表明,SiO_2含量为41.53%～53.59%,富TiO_2为2.14%～3.58%,REE的总量为∑REE=282.3×10~(-6),轻稀土元素较重稀土元素富集[(La/Yb)_N=8.34],略显Eu正异常(Eu~*=1.13～1.25),具有较高的Ti/Y(300～622)和Zr/Y(3.8～7.4)比值.大离子亲石元素Rb、Ba和K相对富集,无明显的Nb和Ta异常,其Nb和Ta值为N-MORB标准值的10～20倍,为典型洋岛玄武岩(OIB)地球化学特征.锆石SHRIMP Ⅱ U-Pb测年获得能干上和蚂蝗峡基性岩块体分别形成于446Ma和455Ma.这些资料进一步表明,奥陶纪时期南秦岭存在着古洋盆.

Identification of the Ordovician oceanic island basalts and their tectonic significance of the Dabao Formation in southern Qinling: Constraints from geochemistry and geochronology of oceanic island basalt

The Qinling orogen is divided into northern and southern Qinling tectonic belts by Shangxian-Danfeng tectonic melange. The Dabao Formation is located in the southern Qinling orogen and is composed of grey and dark grey mudstone, siltstone, silicic slate matrix and chert, limeatone and basalt blocks. The Nengganshang and Mahuangxia basalt blocks are contained within black mudstone matrix in Dabao Formation. Based on geochemical analysis, the Nengganshang and Mahuangxia basalt blocks have similar geochemical characteristics to those of OIB, with high SiO_2 (41.53% ～ 53.59%), high TiO_2 (2.14% ～3.58%), Fe_2O_3~T (10.23% ～ 15.64%), high REE (282.3 × 10~(-6)) contents, high Ti/Y (300 ～ 622) , Zr/Y (3.8 ～ 7.4), and show enrichment light REE[ (La/Yb)_N = 8.34], weak Eu positive anomaly (Eu~* =1.13 ～1.25), without abnormity in Nb and Ta, and enrichment in Rb, Ba and K. The zircon SHRIMPⅡ U-Pb dating results yielded ages of 446Ma and 455Ma in Nengganshang and Mahuangxia basalt blocks. The data suggests the existence of an ocean basin in south Qinling during Ordovician.