LIU Xinshu1,2,WANG Wei-(RZ)1,2,BAO Hong1,2,GONG Tingnan1,2,3,ZHAN Liqing1,4,LIU Xiaochun1,2,ZHAO Yue1,2
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摘要: 东南极普里兹构造带与罗迪尼亚和冈瓦纳超大陆的演化密切相关,但其演化历史还存在争议。为了进一步完善该构造带的演化历史,采用矿物学、岩石学及相平衡模拟等方法对普里兹构造带米洛半岛尖晶石−堇青石麻粒岩进行了研究。结果表明其主期矿物组合为尖晶石+堇青石+黑云母+夕线石+少量石榴子石+钾长石+钛铁矿,温压条件为870~910 ℃、6.4~6.9 kbar,晚期退变至810~820 ℃、4.9~5.3 kbar,并暗示了更高的峰期变质条件(T>910℃,P>6.9 kbar)。CL图像显示锆石具有明显的核−幔−边结构,LA−ICP−MS 锆石U–Pb年代学分析显示核部年龄主要集中在613±7 Ma到877±9 Ma,最大值916±11 Ma,比典型的格林维尔期年龄年轻。锆石边部加权平均年龄为526±8 Ma,其Th/U比值范围较大(0.06~1.23),多数Th/U比值较高(>0.1),应代表峰后冷却结晶阶段。尖晶石−堇青石麻粒岩记录了中低压/高温—超高温的变质条件,结合区域已有资料,可能具有顺时针轨迹,其变质演化历史可能反映了碰撞造山之后的伸展阶段,推测与冈瓦纳超大陆的聚合过程有关。
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关键词:
- 东南极 /
- 尖晶石–堇青石麻粒岩 /
- P–T轨迹 /
- 变质时间 /
- 相平衡模拟
Abstract: The East Antarctic Shield records various geological processes from Archean to Phanerozoic. The Prydz Bay belt was closely related to the evolution of the Rodinia and Gondwana supercontinents, but the geological history of the region remains unclear. The mineral assemblage of the spinel-cordierite granulite from the Mirror Peninsula in the Prydz Bay belt includes spinel, cordierite, biotite, sillimanite, minor garnet, K-feldspar and ilmenite. Combined with paragenetic mineral assemblages and mineral compositions, the phase equilibrium modelling constrains peak pressure-temperature(P-T) conditions to 870~900°C, 6.3~6.9kbar, and retrograde P-T conditions to 810~820°C, 4.9~5.3kbar. Based on reintegrated spinel compositions, the formation temperature of spinel was constrained to 750~810℃, comparable with those retrograde conditions constrained using phase equilibrium modelling. Zircon grains commonly show core-mantle-rim structures in cathodoluminescence (CL) images. The core domains show a wide age range between 613±6.6Ma to 877±8.6Ma with a maximum age of 916±10.9Ma, which are younger than typical age of Greenvillian events. The bright rims of zircon grains yield a weighted mean age of 525.7±8.2Ma with a wide range of Th/U (0.06~1.23), mostly higher than 0.1. Combined with previous results, the medium- to low-pressure granulite facies metamorphism as revealed by the spinel-cordierite granulite could be interpreted to form in an orogenic process from collisional to subsequent post-collisional stage, possibly associated with the assembly of the Gondwana supercontinent.
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