P–T–t evolution of pelitic gneiss from the basement underlying the Northwestern Ordos Basin, North China Craton, and the tectonic implications
P–T–t evolution of pelitic gneiss from the basement underlying the Northwestern Ordos Basin, North China Craton, and the tectonic implications
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DOI：
10.1016/j.precamres.2016.01.030
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发表时间：
2016-05
影响因子：
3.8
通讯作者：
Longlong Gou;Chengli Zhang;Michael Brown;P. Piccoli;Hong-Bing Lin;Xinshan Wei
中科院分区：
文献类型：
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作者：
Longlong Gou;Chengli Zhang;Michael Brown;P. Piccoli;Hong-Bing Lin;Xinshan Wei
利用岩相学、相平衡模拟和原位(U-Th)-Pb独居岩年代学方法，研究了鄂尔多斯盆地西北部祁坛1井近井底泥质片麻岩的变质演化过程。M1-M3期次为：基质中的金红石残片和黑云母+钾长石+斜长石+硅线石+石英包裹体(M1)；石榴石上的无包裹体以钛铁矿(部分取代金红石)和黑云母+钾长石+斜长石+硅线石+石英(M2)为代表；组合中的堇青石出现：黑云母+硅灰石+云母+石英。最终的亚固相线矿物组合为石榴石+黑云母+钾长石+斜长石+硅线石+堇青石+钛铁矿+石英(M4)。矿物组合演化的P-T条件(M1、→、M4)受构造于Ap-T假剖面的制约。P-T-T路径是顺时针方向的，在从∼8.8kbar(∼769℃)到5.5kbar(785℃)的近等温减压过程中，涉及轻微的加热，然后在固相线上进行近等压冷却。两个独居石核心和三个具有相似年龄组的未分带颗粒获得了1.96-1.94aGa的可靠年龄，解释了晚期变质作用(M1-M2)的时间。此外，一个边缘和两个未分带的独居石颗粒具有相似的枣树种群，产生了1.9-1.88Ga的稳健年龄，解释为解压后固相线缓慢冷却的时间(M3、→和M4)。根据这些结果，泥质片麻岩被解释为在银山地体与鄂尔多斯地体碰撞过程中地壳增厚(M1)，后者与其北缘孔兹岩带原岩(今坐标)形成的。这种碰撞在约1.95Ga时达到顶峰，随后在碰撞后伸展过程中，减压(M2-M3)和缓慢冷却(M4)达到约1.89TGa。本研究确定的P-T-T路径与孔兹岩带HP/MP麻粒岩的路径相似，证实它们经历了共同的构造变质历史。P-T-TPath记录了从峰值压力下∼75°C/kbar的表观温度梯度到减压后∼150°C/kbar的演化过程。再加上鄂尔多斯盆地基底中古元古代岩浆作用的产出，这些特征表明鄂尔多斯地体是上板块的会聚边缘，孔兹岩带代表其北缘的弧前沉积。与俯冲板块的一部分--银山地体的碰撞导致了浅层板片断裂和双侧热碰撞的发展，形成了宽阔的高原状热造山带，这与Sizova等人的数值模型中的结果类似。(2014)。
Using petrography, phase equilibria modeling and in situ (U–Th)–Pb monazite geochronology, we show that pelitic gneiss from close to the bottom of the Qitan1 borehole in the northwest of the Ordos Basin records a four stage metamorphic evolution. The M1–M3 stages, which were suprasolidus, are represented by: rutile relicts in the matrix and biotite + K-feldspar + plagioclase + sillimanite + quartz inclusions in the cores of garnet porphyroblasts (M1); inclusion-free mantles on garnet with ilmenite (partially replacing rutile) and biotite + K-feldspar + plagioclase + sillimanite + quartz in the matrix (M2); and, the appearance of cordierite in the assemblage biotite + K-feldspar + plagioclase + sillimanite + ilmenite + quartz (M3). The final subsolidus mineral assemblage is garnet + biotite + K-feldspar + plagioclase + sillimanite + cordierite + ilmenite + quartz (M4). TheP–Tconditions for the mineral assemblage evolution (M1 → M4) are constrained by aP–Tpseudosection constructed in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O2chemical system. TheP–T–tpath is clockwise involving slight heating during near isothermal decompression from ∼8.8 kbar at ∼769 °C to 5.5 kbar at 785 °C followed by close-to-isobaric cooling across the solidus. Two cores of monazite and three unzoned grains that have similar populations of dates yield robust ages of 1.96–1.94 Ga, interpreted to date the timing of late prograde-to-peak metamorphism (M1–M2). In addition, one rim and two unzoned monazite grains with similar populations of dates yield robust ages of 1.90–1.88 Ga, interpreted to date the timing of post decompression slow cooling across the solidus (M3 → M4). On the basis of these results, the pelitic gneiss is interpreted to have formed by crustal thickening (M1) during collision between the Yinshan and Ordos Terranes, the latter with the Khondalite Belt protoliths along its northern margin (present co-ordinates). This collision culminated at ca. 1.95 Ga and was followed by decompression (M2–M3) and slow cooling (M4) to ca. 1.89 Ga during post-collisional extension. The similarity between theP–T–tpath determined in this study and those from HP/MP granulites in the Khondalite belt confirms that they experienced a common tectono-metamorphic history. TheP–T–tpath records an evolution from an apparent thermal gradient of ∼75 °C/kbar at peak pressure to ∼150 °C/kbar after decompression. Together with the occurrence of mid-Paleoproterozoic magmatism in the basement of the Ordos Basin, these features indicate that the Ordos Terrane was the upper plate at a convergent margin with the Khondalite Belt representing the forearc sediments along its northern edge. Collision with the Yinshan Terrane, part of the subducting plate, led to shallow slab breakoff and the development of a two-sided hot collision and formation of a wide plateau-like hot orogen, similar to those in the numerical models of Sizova et al. (2014).