兴安地块南段哈达地区中晚侏罗世侵入岩成因及对蒙古-鄂霍茨克洋演化的制约

Petrogenesis of the Middle to Late Jurassic diorite and granite from Hada area in the Southern Xing an Block and its restriction on the evolution of the Mongol-Okhotsk Ocean

中晚侏罗世是东北地区构造演化中的一个重要转折时期,但该时期岩浆岩出露相对少,研究程度低。文章对兴安地块南段的中晚侏罗世侵入岩首次进行了LA-ICP-MS锆石U-Pb年代学、岩石地球化学研究,分析岩石成因与地质意义,进一步限定蒙古-鄂霍茨克洋在兴安地块南段的闭合时限。研究表明:侵入岩包括花岗斑岩、闪长玢岩2种岩性,成岩年龄分别为(167.0±0.5)Ma、(160.0±0.7)Ma,为中晚侏罗世岩浆作用的产物;岩石具有富钠(Na 2O含量分别为4.57%~4.97%、3.21%~3.98%)、准(弱)铝质(A/CNK分别为1.00~1.07、0.93~1.10)的特点,属于高钾钙碱性系列的I型花岗岩;具有富集Rb、Th和K等大离子亲石元素,亏损Ta、Nb、P和Ti等高场强元素,稀土元素总量中等(ΣREE分别为(90.5~153.1)×10^(-6)、(111.6~128.8)×10^(-6)),轻稀土元素相对重稀土元素分馏明显(LREE/HREE值分别为6.74~10.26、5.14~8.84,(La/Yb)N分别为5.61~9.58、4.92~10.38),中低负铕异常(δEu分别为0.58~0.68、0.83~1.02)的微量元素特征。推断侵入岩成因很可能与蒙古-鄂霍茨克洋的闭合有关,形成于加厚陆壳坍塌或拆沉作用下的构造背景,即挤压向伸展构造转换阶段。

The Middle to Late Jurassic is an important transitional period in the tectonic evolution of Northeast China,the study of magmatic rocks during this period is relatively limited due to its rare exposure.In this study,LA-ICP-MS zircon U-Pb geochronology and rock geochemistry were used for the first time to investigate the Middle to Late Jurassic intrusive rocks located in the southern part of the Xing an Block,analyzing its petrogenesis and geological significance,and further constraining the closure timing of the Mongolia-Okhotsk Ocean in the southern Xing an Block.The study shows that the intrusive rocks include two types of rock:granite porphyry and diorite porphyrite,with crystallization ages of(167.0±0.5)Ma and(160.0±0.7)Ma,respectively,indicating that they are products of Middle to Late Jurassic magmatic activities.Two rocks belong to the high-K calc-alkaline I-type series,characterized by Na 2O contents ranging from 4.57%to 4.97%and 3.21%to 3.98%,and weak aluminum saturation with A/CNK ratios ranging from 1.00 to 1.07 and 0.93 to 1.10.They also have characteristics of enrichment in large ion lithophile elements(Rb,Th and K),depletion in high field-strength elements(Ta,Nb,P and Ti),moderate rare earth element contents(ΣREE)of(90.5~153.1)×10^(-6) and(111.6~128.8)×10^(-6),significant fractionation between light and heavy rare earth elements with LREE/HREE ratios of 6.74~10.26 and 5.14~8.84,(La/Yb)N of 5.61~9.58 and 4.92~10.38,and middle negative Eu anomalies(δEu)ranging from 0.58 to 0.68 and 0.83 to 1.02.These results can be inferred that the petrogenesis of the intrusive rocks is likely related to the closure of the Mongolia-Okhotsk Ocean,and they formed in a tectonic background of crustal thickening collapse or detachment,during the transition from compression to extension tectonics.