黑龙江省西北部晚古生代I-A型花岗岩的成因及构造意义

Petrogenesis and Tectonic Implication of Late Paleozoic I-A Type Granites in the Northwest Heilongjiang Province

本文以黑龙江省西北部大兴安岭塔源镇和小兴安岭二站乡地区花岗岩为研究对象,通过岩石学、U-Pb年代学及地球化学分析,揭示了古亚洲洋在该地区的俯冲—碰撞过程。塔源黑云母二长花岗岩U-Pb年龄为308 Ma,为晚石炭世侵入体,具有高硅、高碱的特征,富集轻稀土元素和大离子亲石元素,亏损重稀土元素和高场强元素,εHf(t)为0.8~2.5,地球化学特征显示其来源于增生的下地壳物质;二站乡二长花岗岩、碱长花岗岩U-Pb年龄分别为305和293 Ma,同样为高硅、高碱系列,富集轻稀土元素和大离子亲石元素,亏损重稀土元素和高场强元素,地球化学特征显示其来源于新生地壳。晚石炭世花岗岩为Ⅰ型花岗岩,早二叠世花岗岩为A型花岗岩。利用锆石δEu定量限定了塔源二长花岗岩和二站乡二长花岗岩的形成深度,分别为38.8和34.7 km,而碱长花岗岩形成深度为28.7 km,暗示晚石炭世至早二叠世该地区经历了地壳的拉张减薄,记录了古亚洲洋的俯冲消亡与之后的后碰撞伸展作用。

Petrology, geochemistry and U–Pb geochronology of the Great Xing’an Range Tayuan and the Little Xing’an Range Erzhan Country granites in the northwest Heilongjiang Province were undertaken to determine the subduction and collisional processes of the Paleo-Asian Ocean. The Tayuan granites are mainly biotite monzogranites with zircon U-Pb age of 308 Ma, which is a Late Carboniferous intrusion, and they are characterized by high silica and total alkali contents, enrichment in light rare earth elements(LREE) and large ion lithophile elements(LILE), depletion in heavy rare earth elements(HREE) and high field strength elements(HFSE). Zircons from the Tayuan monzogranites have εHf(t) values ranging from 0.8 to 2.5, suggesting that the granitic magmas were generated by partial melting of newly-accreted lower crust. The Erzhan Country granites are dominantly monogranite and alkali-feldspar granite with zircon U-Pb ages of 305 Ma and 293 Ma. They have high silica and total alkali contents, and are characterised by enrichment in LREEs and LILE relative to HREE and HFSE, implying that they were generated by the melting of juvenile continental crust. The Late Carboniferous monogranites belong to I-type granite, and the Early Permian alkali-feldspar granite is typical A-type granite. Zircon δEu ratios can be used to estimate the melting depths of granites: The formation depth of Tayuan monzogranite and Erzhan Country monzogranite is 38.8 km and 34.7 km, respectively, while the alkali-feldspar granite formed at a shallow depth of 28.7 km. The crustal thinning during the Late Carboniferous to Early Permian might record the tectonic transition from subduction to post-collisional extension related to the final closure of the Paleo-Asian Ocean.