北秦岭二郎坪岩体锆石U-Pb定年、地球化学特征及其地质意义

二郎坪岩体位于北秦岭构造带的东部,主体岩性为斑状黑云母二长花岗岩,对其开展锆石精确定年和地球化学成因研究有助于深入认识晚中生代北秦岭的深部过程。LA-ICP-MS锆石U-Pb定年结果表明,二郎坪岩体的形成时代为(114.0±0.5)Ma(MSWD=1.5),属于早白垩世晚期。二郎坪岩体富硅、碱和贫钙、镁,属于高钾钙碱性系列;并富集Rb、Th和亏损Nb、Sr、P、Ti,稀土配分模式具有右倾、轻稀土富集、重稀土亏损和负Eu异常的特征,(La/Yb)N范围为8.39～13.73,源区组成和部分熔融程度控制了二郎坪岩体的成分变异。二郎坪岩体形成于厚度约为40km的地壳内,部分熔融源区位于角闪岩相,源区残余相主要矿物为角闪石和斜长石。烟镇区域岩石圈约在130Ma经历拆沉作用,触发二郎坪深部的岩浆底侵作用,使源区岩石部分熔融并与之混合,上侵至地壳浅部最终在约114Ma固结成岩,二郎坪岩体是烟镇区域岩石圈拆沉作用滞后约15Ma的产物。

LA-ICP-MS Zircon U-Pb Dating,Geochemiacal Features and Geological Significance of Erlangping Rock Mass in North Qinling Mountains

Erlangping rock mass is in the east of Northern Qinling Mountains, and composed of porphyritic biotite monzonitic granite. The zircon U-Pb dating and geochemical features of Erlangping rock mass are significant to recognize the deep tectonic evolution of the northern Qinling Mountains in Late Mesozoic. The LA-ICP-MS zircon U-Pb dating results show that the formation age of Erlangping rock body is (114.0±0.5) Ma (MSWD=1.5). Erlangping rock mass is characterized by higher SiO₂ and alkali, and lower MgO and CaO, and compositional spots fall into the high-K calc-alkaline series in the SiO₂-K₂O diagram. Erlangping rock mass are enriched in large ion lithophile elements and depleted of high field-strength elements, which formed peaks of Rb and Th, and troughs of Nb,Sr, P and Ti in spider diagram. In the chondrite-normalized REE patterns, Erlangping rock mass shows the characteristics of LREE-enrichment and HREE depletion with Eu negative anomaly, and the (La/Yb)_N ratios are from 8.39 to 13.73. It suggests that the compositional change of Erlangping rock mass is controlled mainly by source composition and degree of partial melting. The partial melting source of Erlangping rock mass falls in amphibolite-facies with a crust thickness of about 40 km. The residual phases of partial melting source include mainly hornblende and plagioclase. Yanzhen regional lithosphere delamination took place in about 130 Ma, which triggered the deep magma intrusion in Erlangping area, and the later source rock partial melting and mixing with deep magma. They ascended and solidified in about 114 Ma. Erlangping rock mass is the product of delayed magmatism in about 15 Ma in response to the Yanzhen regional lithosphere delamination.