秦岭造山带中部两河口岩体花岗岩锆石U-Pb年代学、地球化学及地质意义

北秦岭两河口岩体位于太白地区，侵位于秦岭群杂岩中，主要岩性为眼球状花岗岩、片麻状花岗岩和二长花岗岩。 本文研究的眼球状花岗岩和片麻状花岗岩的结晶年龄分别为928±19 Ma 和940±12 Ma，岩石中还保留古元古代至中元古代的 继承锆石。眼球状花岗岩含有富铝矿物石榴子石和白云母。岩石的A/CNK 多大于1.1，具有高Si、富铝的特征，属于高钾钙 碱性系列。岩石轻、重稀土分馏明显，具有中等负Eu 异常。岩石富集大离子亲石元素（Rb, Ba, K 等）、亏损高场强元素 （Nb, Ta, Ti 等），具有明显的Ba, P, Sr 负异常。矿物学和地球化学特征显示眼球状花岗岩和片麻状花岗岩为S 型花岗岩。两河 口岩体初始Sr 同位素组成变化大，87Sr/86Sr(t)=0.701067~0.739451，具有较低的εNd(t)=-5.7~-3.3, 两阶段Nd 模式年龄为TDM2= 1.9~2.1 Ga。样品具有高的放射成因Pb 同位素组成，指示两河口岩体是壳源成因岩石，其源岩可能为秦岭群斜长角闪岩和 片麻岩。结合区域地质背景，认为两河口岩体源于新元古代陆壳碰撞晚期的构造转换阶段古老中下地壳的熔融作用，是对 Rodinia 超大陆汇聚事件的响应。

Zircon U-Pb Age,Geochemistry, and Geological Significance of Granites from Lianghekou Pluton,Central Qinling Orogenic Belt

The Lianghekou pluton is located in Taibai area of North Qinling terrane and was emplaced into Qinling Group complex. The pluton is mainly composed of augen granite, gneissic granite and monzogranite. In this study, crystallized ages of 928±19 Ma and 940±12 Ma with the inherited zircons grains ranging from Paleoproterozoic to Mesoproterozoic are obtained from augen granite and gneissic granite, respectively. Augen granite contains aluminum minerals of garnet and muscovite. Most granites show the characteristics of A/ CNK＞1.1, high silica and aluminum content as well as high-K calc-alkaline series. Chondrite-normalized REE patterns show a rightward inclination with medium negative Eu anomalies. The trace element geochemistry is characterized by enrichment in LILEs (such as Rb, Ba, K, et al.), depletion in HFSEs (such as Nb, Ta, Ti, et al.) and obviously negative anomalies of Ba, P, Sr. Mineralogical and geological characteristics indicate that the gneissic granites and augen granites of Lianghekou pluton are typical S-type granites. Lianghekou granites show large range of initial 87Sr/86Sr(t) ratios of 0.701067-0.739451, lower εNd(t) values varied from -5.7 to -3.3 with two-stage Nd modal ages of TDM2=1.9-2.1 Ga and highly radiogenic Pb composition, indicating a crustal magmatic origin and the source rocks may be amphibolite and gneiss of Qinling Group. Combined with the regional geological background, we conclude that granites of Lianghekou pluton originated from melting of ancient middle-lower crust under tectonic transition stage of late continental crust collision during Neoproterozoic in response to the assembly of Rodinia supercontinent.