Generation of post-collisional normal calc-alkaline and adakitic granites in the Tongbai orogen, central China
Generation of post-collisional normal calc-alkaline and adakitic granites in the Tongbai orogen, central China
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桐柏造山带碰撞后正钙碱性和埃达克质花岗岩的生成
DOI：
10.1016/j.lithos.2017.11.033
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发表时间：
2018
期刊：
影响因子：
3.5
通讯作者：
Yuanbao Wu
中科院分区：
文献类型：
--
作者：
Wen-Xiang Zhang;Liu–Qin Zhu;Hao Wang;Yuanbao Wu
碰撞后花岗岩一般是造山伸展过程中陆壳部分熔融形成的。碰撞造山带中继埃达克质花岗岩之后出现正常钙碱性花岗岩，常被认为是构造体制由挤压向伸展转变的标志，但这一观点一直存在争议。本文对桐柏造山带和鸡公山造山带碰撞后花岗岩进行了锆石U-Pb年龄、Hf-O同位素、全岩主量元素、微量元素和锶-Nd同位素的综合研究。桐柏和鸡公山岩体的锆石U-Pb年龄分别为140 Ma和135 Ma，表明它们是碰撞后花岗岩。这些花岗岩为高钾钙碱性系列，偏铝至弱过铝，A/CNK比值为0.85~1.08。桐柏片麻状花岗岩为正常钙碱性花岗岩，具有可变的SiO_2(61.93~76.74wt%)、Sr/Y(2.9~38.9)和(La/Yb)N(1.7~30.1)比值，Eu负异常(0.41~0.92)。它们具有相对较高的初始锶同位素比值0.707571~0.710317，较低的εNd(T)(−15.74~−11.09)和εHf(T)(−17.6~−16.9)。它们的模式年龄分别为2.2~1.8Ga和2.3~2.2Ga。鸡公山花岗岩具有较高的SiO_2(66.56~72.11wt%)、(La/Yb)N(27.4~91.4)和(La/Y)(30.1~182.0)比值，Eu异常不明显(0.73~1.00)，属埃达克质花岗岩。它们的ISR=0.707843-0.708366，εND(T)=−19.83到−17.59，εHf(T)=−26.0到−23.5。它们的Nd和Hf模式年龄分别为2.5~2.4Ga和2.8~2.6Ga。桐柏花岗岩和鸡公山花岗岩具有类似地幔的锆石δ18O值(5.17~5.46‰)。这些地球化学特征表明桐柏花岗岩和鸡公山花岗岩分别来自古元古代陆壳和太古代陆壳的部分熔融。分离结晶作用影响桐柏片麻状花岗岩的地球化学成分，而鸡公山花岗岩的成分主要受部分熔融的控制。鸡公山花岗岩的埃达克岩特征或继承自其源岩，或归属于其残留物中更多的石榴石。大量的碰撞后花岗岩可能是由于加厚的桐柏造山根坍塌而形成的。桐柏造山带中正常桐柏片麻状花岗岩的形成略早于鸡公山埃达克质花岗岩，表明碰撞后环境下埃达克质花岗岩向正常钙碱性花岗岩的构造转换并不是必需的。因此，这项研究提供了另一个例子，支持碰撞造山带加厚的造山根坍塌导致不同成分的大量碰撞后花岗岩的破裂。
Post-collisional granites are generally generated by partial melting of continental crust during orogenic extension. The occurrence of normal calc-alkaline granites following adakitic granites in a collisional orogen is frequently supposed as a sign of tectonic regime transition from compression to extension, which has been debated yet. In this paper, we present a comprehensive study of zircon U–Pb ages, Hf–O isotopes, as well as whole-rock major and trace elements and Sr–Nd isotopes, for Tongbai and Jigongshan post-collisional granitic plutons in the Tongbai orogen. Zircon U–Pb dating yields intrusion ages of ca. 140 and 135 Ma for the Tongbai and Jigongshan plutons, respectively, suggesting they are post-collisional granites. These granites are high-K calc-alkaline series, metaluminous to weakly peraluminous with A/CNK ratios of 0.85–1.08. The Tongbai gneissic granites are normal calc-alkaline granite, having variable SiO2 (61.93–76.74 wt%) and Sr/Y (2.9–38.9) and (La/Yb)N (1.7–30.1) ratios with variably negative Eu anomalies (0.41–0.92). They have relatively high initial.Sr isotope ratios of 0.707571 to 0.710317, and low εNd(t) (−15.74 to −11.09) and εHf(t) (−17.6 to −16.9) values. Their Nd and Hf model ages range from 2.2 to 1.8 Ga and 2.3 to 2.2 Ga. On the contrary, the Jigongshan granites show higher SiO2 (66.56–72.11 wt%) and Sr/Y (30.1–182.0) and (La/Yb)N (27.4–91.4) ratios with insignificant Eu anomalies (0.73–1.00), belonging to adakitic granite. They have Isr = 0.707843–0.708366, εNd(t) = −19.83 to −17.59, and εHf(t) = −26.0 to −23.5. Their Nd and Hf model ages vary from ca. 2.5 to 2.4 Ga and ca. 2.8 to 2.6 Ga. The Tongbai and Jigongshan granites are characterized by mantle-like zircon δ18O values (5.17–5.46‰). These geochemical features suggest that the Tongbai and Jigongshan graniteswere derived frompartialmelting of Paleoproterozoic and Archean continental crust, respectively. Fractional crystallization affected the geochemical compositions of the Tongbai gneissic granites, while the compositions of the Jigongshan graniteswere mainly controlled by partialmelting. The adakitic signatures of the Jigongshan graniteswere either inherited fromtheir source or ascribed to more garnet in their residues. The voluminous post-collisional granites might formby the collapse of the thickened Tongbai orogenic root. The normal Tongbai gneissic granites occurred slightly earlier than the Jigongshan adakitic granites in the Tongbai orogen, suggesting that it is not amandate to sign the tectonic transition from adakitic to normal calc-alkaline granites in post-collisional settings. Therefore, this study provides another example supporting the burst of voluminous post-collisional granites with different compositions as a consequence of the collapse of the thickened orogenic roots of collisional orogens.
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