Geochemistry and Zircon U‐Pb Dating of Changshagou Adakite from the South Altyn UHPM Terrane: Evidence of the Partial Melting of the Lower Crust
Geochemistry and Zircon U‐Pb Dating of Changshagou Adakite from the South Altyn UHPM Terrane: Evidence of the Partial Melting of the Lower Crust
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DOI：
10.1111/1755-6724.12311
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发表时间：
2014-10
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通讯作者：
Lei Kang;Liang Liu;Chao Wang;Yuting Cao;Wenqiang Yang;Yawei Wang;Xiaoying Liao
中科院分区：
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作者：
Lei Kang;Liang Liu;Chao Wang;Yuting Cao;Wenqiang Yang;Yawei Wang;Xiaoying Liao
长沙沟埃达克岩是南阿尔金超高压变质(UHPM)地体中段的露头，含中钾碱性和弱过铝组分(SiO_2=66.79%~68.65%，Al_2O_3=17.48%~18.31%，K2O+Na2O=6.32%~6.88%，K2O/Na2O=0.25~0.33，A/CNK=1.01~1.06)。这个露头也富含大离子亲石元素，但亏损的高场强元素(HFSE)显示明显的负Nb、Ta和Ti异常。稀土配分模式为Eu正异常(δEu=1.15~1.31)，轻稀土较重稀土弱富集(LREE/HREE=1.02~4.20)。实验结果和几个特征，包括相对较低的Nb/Ta比值(6.03~8.45)和较高的锶、锶/Y、(La/Yb)N和低Y和Yb特征，表明源区存在残余石榴石而不存在斜长石。结果表明，埃达克岩可能在1.2~1.5 Gpa的压力和约900°C的温度下形成。SiO_2-Mg#和SiO_2-Mg_2O图解表明，岩石是由加厚的下陆壳部分熔融形成的。LA-ICPMS原位U-Pb定年获得两组年龄：503.1±1.7Ma(核)和453.1±3.0Ma(缘)。岩芯和边缘的Th/U比值分别为0.11~0.40和0.03~0.07。结合锆石CL图像特征、Th/U比值和前人对区域超高压变质岩的研究，认为埃达克岩形成于503.1±1.7Ma，是阿尔金深俯冲陆壳在453.1±3.0Ma裂解的结果。
Changshagou adakite, an outcrop in the middle segment of the South Altyn Tagh ultra‐high pressure metamorphism (UHPM) terrane, contains medium‐K cal‐alkaline and weakly peraluminous compositions (SiO2= 66.79% to 68.65%, Al2O3= 17.48% to 18.31%, K2O + Na2O = 6.32% to 6.88%, K2O/Na2O = 0.25 to 0.33, A/CNK = 1.01 to 1.06). This outcrop is also enriched with large ion lithophile elements but with depleted high‐field strength elements (HFSE) showing clearly negative Nb, Ta, and Ti anomalies. REE distribution patterns show a positive anomaly of Eu (δEu = 1.15 to 1.31) and weakly enriched with LREE compared with HREE (LREE/HREE = 1.02 to 4.20). Experimental results and several characteristics, including relatively low Nb/Ta ratios (6.03 to 8.45) and high Sr, Sr/Y, (La/Yb)N and low Y and Yb, which indicate the presence of residual garnet and the absence of plagioclase in the source region, show that adakite may form at a pressure ranging from 1.2 GPa to 1.5 GPa and at a temperature of approximately 900°C. Low Cr, Ni, and Mg# values, trace element patterns, and SiO2–Mg# and SiO2–MgO diagrams indicate that rocks are formed by the partial melting of a thickened lower continental crust LA‐ICP‐MS in situ U‐Pb dating yields two group ages: 503.1 ± 1.7 Ma (core) and 453.1 ± 3.0 Ma (rim). The Th/U ratios of the core and the rim are 0.11 to 0.40 and 0.03 to 0.07, respectively. Considering the zircon CL image characteristics, Th/U ratios, and previous studies on regional UHPM rocks, adakite formed at 503.1 ± 1.7 Ma and underwent a tectothermal event as a result of the break‐off of the Altyn deep subducted continental crust at 453.1 ± 3.0 Ma.