蚌埠荆山地区变质P-T条件评价与锆石U-Pb定年分析

The analysis of P-T conditions and zircon U-Pb dating in Jingshan area, Bengbu

本次研究区荆山位于蚌埠隆起区西部,主要由荆山岩体和太古代五河杂岩(庄子里组)组成。前人主要对荆山岩体进行了大量详实的年代学研究,认为其形成于晚侏罗世(160.2±1.3 Ma),继承锆石记录了(217.1±6.6 Ma)三叠纪变质改造作用,但是对该地区变质岩石学和年代学研究相对缺乏。为此,本文对荆山岩体南侧的角闪岩(WS002-2)、含石榴子石花岗片麻岩(WS004-2)和斜长角闪岩(WS003-2)以及其内部的黑云斜长角闪岩(WS007-1)夹块进行了细致的变质P-T条件分析和锆石U-Pb年代学分析。样品WS002-2因矿物组合简单,适用的温压计有限,其获得的P-T值最低:-T=617±13℃和P-=0.39±0.02 GPa;WS004-2获得的变质P-T条件为T-=545±8℃~549±8℃和P-=0.92±0.02 GPa~1.03±0.02 GPa;WS003-2和WS007-1获得的P-T条件较为一致,二者的平均P-T条件范围为T-=650±11℃~675±10℃和P-=0.64±0.06 GPa~0.70±0.06 GPa。为了进一步细化变质时限,本文对样品WS004-1(花岗片麻岩)和WS007-1进行了锆石U-Pb年代学分析,前者记录了3期明确的地质事件:(1) 2 808±65 Ma;(2) 2 657±64 Ma~2 533±67 Ma;(3) 1 873±80 Ma~1 802±75 Ma,应分别指示了初始陆壳形成时限、华北克拉通~2.5 Ga和1.9~1.8 Ga经历的高级变质和退变质作用;后者记录了4期明确的地质事件:(1) 157±4 Ma;(2) 244±6 Ma~212±5 Ma;(3) 480±15 Ma~336±8 Ma;(4) 774±19 Ma~579±14 Ma。其中,变质年龄157±4 Ma仅1颗,应记录了荆山地区经历了晚侏罗世变质作用;年龄(2)应与蚌埠隆起区三叠纪超高压地幔/下地壳拆沉作用相关;年龄(3)和年龄(4)与华北克拉通普遍经历的古生代—中生代变质改造、大规模岩浆活动以及新元古代岩浆活动较为一致。本次研究进一步丰富了荆山地区的变质岩石学和年代学结果,初步将蚌埠隆起区划分为4个变质P-T区间,其形成时限与变质年代学研究结果具有良好的一致性。

Jingshan is located in the west of Bengbu uplift and mainly composed of migmatitic granite and Archean Wuhe complex(Zhuangzili Group). Many detailed chronology studies on Jingshan migmatitic granite have been conducted before, and reckoned that it was formed in the Late Jurassic(160.2±1.3 Ma) and underwent(217.1±6.6 Ma) Triassic metamorphic transformation. However, there is a lack of research on metamorphic petrology and chronology in this area. Therefore, the studying focused on the metamorphic P-T conditions and zircon U-Pb chronology analysis and were carried out for amphibolite(WS002-2), garnet bearing granitic gneiss(WS004-2), plagioclase amphibolite(WS003-2) and biotite-plagioclase amphibolite(WS007-1) from southern side Wuhe complex of Jingshan migmatitic granite.WS002-2 obtained the lowest P-T value due to its simple mineral group and thermo-barometer limitation:T-= 617±13 ℃ and P-= 0.39±0.02 GPa. The average P-T value of WS004-2 is T-=545±8 ℃ ~549±8 ℃ and P-= 0.92±0.02 GPa~1.03±0.02 GPa. The P-T conditions of WS003-2 and WS007-1 are relatively consistent, the average values are T-= 675±10 ℃~650±11 ℃ and P-= 0.64±0.06 GPa~0.70±0.06 GPa respectively. Zircon U-Pb chronology analysis was carried out on WS004-1(granitic gneiss)and WS007-1, the results revealed that WS004-1 recorded 3 geological events:(1)2 808±65 Ma,(2)2 657±64 Ma ~ 2 533±67 Ma and(3)1 873±80 Ma~1802±75 Ma, they should be indicated the time of continental crust formation, the prograde metamorphism and retrograde metamorphism of the North China Craton ~2.5 Ga and1.9 ~ 1.8 Ga respectively. Sample WS007-1 recorded 4 geological events :(1)157±4 Ma,(2) 244±6 Ma ~212±5 Ma,(3) 480±15 Ma~336±8 Ma and(4) 774±19 Ma~579±14 Ma.Among them, the metamorphic age of 157±4 Ma only 1 result, which should record the Late Jurassic metamorphism in Jingshan area. The age-(2) is related to the Triassic UHP mantle/lower crust delamination in Jinshan. Age-(3) and age-(4) are consistent with the Paleozoic-Mesozoic metamorphic transformation, large scale magmatic activity and Neoproterozoic activity generally experienced in the North China Craton. The study further enriched the metamorphic petrology and chronology analysis in Jingshan and preliminarily divided Bengbu uplift into 4 metamorphic P-T value sections, the P-T conditions and metamorphic chronology results showed good consistency.