北秦岭板山坪岩体锆石U-Pb年代学及岩石成因研究

板山坪岩体是北秦岭二郎坪群中的侵入岩。为了查明该岩体的成因，对该岩体进行了锆石U-Pb年代学、锆石原位Hf同位素研究以及矿物化学分析等方面的研究。研究结果表明，板山坪岩体岩性组成主要为石英闪长岩和花岗闪长岩，花岗闪长岩内部存在暗色包体。本次研究获得板山坪石英闪长岩锆石U-Pb年龄为442.7～432.2 Ma，花岗闪长岩锆石U-Pb年龄为436.8～432.7 Ma，暗色包体锆石U-Pb年龄为437.6 Ma。锆石¹⁷⁶Hf/¹⁷⁷Hf值为0.282 737～0.282 736，ε_Hf（t）值集中分布在8.4～9.4之间，二阶段Hf模式年龄（T_DM2）在876～832 Ma之间。石英闪长岩结晶温压分别为673 ℃～745 ℃和0.19～0.54 GPa，花岗闪长岩结晶温压分别为657 ℃～730 ℃和0.48～0.96 GPa，暗色包体结晶温压分别为680 ℃～734 ℃和0.69～1.65 GPa。综合分析认为板山坪岩体为复式岩体，两期结晶年龄分别为496～487 Ma和442～432 Ma。岩石来源于地幔分离出来的新生下地壳。

Zircon U-Pb chronology and petrogenesis of the Banshanping pluton in the North Qinling area

The Banshanping pluton is an intrusive rock in the Erlangping Group of the North Qinling. The lithology is mainly quartz diorite and granodiorite. We present zircon U-Pb isotope chronology, mineral chemical analysis and Hf isotopes to study the genesis. The pluton is mainly composed of quartz diorite and granodiorite, and dark inclusions exist in granodiorite. The zircon U-Pb age of the Banshanping quartz diorite is 442.7～432.2 Ma, the zircon U-Pb age of the granodiorite is 436.8～432.7 Ma, and the dark inclusion zircon U-Pb age is 437.6 Ma. The Hf isotopes results show that the ¹⁷⁶Hf/¹⁷⁷Hf values of the zircons are between 0.282 737～0.282 736, and the ε_Hf（t） values of the zircons are between 8.4 and 9.4, and the two stage Hf model age ranging from 876～832 Ma. The crystallization temperatures and pressures of quartz diorite are 673 ℃～745 ℃ and 0.19～0.54 GPa, granodiorite 657 ℃～730 ℃ and 0.48～0.96 GPa, dark inclusion 680 ℃～734 ℃ and 0.69～1.65 GPa, respectively. According to the comprehensive analysis, the Banshanping pluton is a complex rock mass, and the two phases of crystallization age are 496～487 Ma and 442～432 Ma, respectively. The rock is derived from the new lower crust that is separated from the mantle.