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Magmatic Water Content and Crustal Evolution Control on Porphyry Systems: Insights from the Central Asian Orogenic Belt
Journal of Petrology ( IF 3.9 ) Pub Date : 2021-03-05 , DOI: 10.1093/petrology/egab021
Chao Wu 1, 2 , Huayong Chen 1, 2 , Yongjun Lu 3, 4
Affiliation  

Porphyry systems, the most important reserves of Cu and Mo with significant Au, are genetically linked to the emplacement of hydrous and oxidized intermediate to acidic magmas, in response to temporal and geochemical evolution of crust in orogenic terranes. In this study, comprehensive whole-rock and zircon geochemical and isotopic datasets of intermediate to acid igneous rocks were integrated to characterize the crustal evolution and metallogeny of porphyry deposits in the Central Asian Orogenic Belt (CAOB). The ore-forming porphyries of Cu ± Au ± Mo and Mo deposits have higher but largely overlapped ΔFMQ (the proxy for oxygen fugacity) with those of the barren igneous rocks. However, the ore-forming porphyries of Cu ± Au ± Mo deposits are characterized by distinctly higher whole-rock V/Sc and zircon Eu/Eu* (both are proxies for water content) than barren rocks. Furthermore, the V/Sc ratios positively correlate with the Cu tonnages of calc-alkali porphyry Cu deposits, suggesting that magmatic water contents may yield the first-order control on metal endowment. The general decrease of V/Sc from the Paleozoic to Mesozoic, combined with the negative correlation of V/Sc with K2O and SiO2, also indicates the gradual evolution of crust in CAOB clearly controls the end members of porphyry-type systems (i.e., Cu‒Au and Mo deposits). The crustal residence age (TRes, the time difference between Nd depleted-mantle model age and the crystallization age) of c. 500 Ma from the Nd isotopes is proposed as the threshold distinguishing porphyry Cu ± Au ± Mo (TRes < 500 Ma) and Mo deposits (TRes > 500 Ma) in the CAOB. The coupled zircon Hf isotopes and crustal thickness reveal that the fundamental crustal architecture in the eastern and western CAOB had been built by the Late Permian and Late Carboniferous, respectively, highlighted by the converging trends of εHf(t) commencing at c.250 Ma in the eastern segment and c.300 Ma in the western segment of CAOB, indicating reworking and homogenizing of juvenile crust after collision. In the eastern CAOB, porphyry Cu ± Au ± Mo deposits were formed by juvenile materials in thin island arcs, while porphyry Mo deposits were formed by reworked materials in the thickened orogenic crust after c.250 Ma. In the western CAOB, porphyry Cu deposits in the Balkhash region during the Late Carboniferous were formed in thickened continental crust (generally > 40 km), genetically linked to the culmination of world-wide magmatic addition rates (MARs) triggered by accelerated production of the juvenile crust, in contrast to the porphyry Cu ± Au ± Mo deposits formed in thin island arc (generally < 40 km) during the Early Paleozoic. This study tests the zircon ΔFMQ as proxy for fO2, and zircon Eu/Eu* and whole-rock V/Sc ratios as proxy for water content. It highlights that whole-rock V/Sc ratio is a favorable index for the Cu tonnages of porphyry Cu ± Au ± Mo deposits, and that the distinct porphyry-type mineralization in the CAOB is controlled by the crustal evolution reflected by crustal composition and thickness.

中文翻译:

斑岩系统的岩浆含水量和地壳演化控制:来自中亚造山带的见解

斑岩系统是含大量金的铜和钼的最重要储量,在成因上与酸性岩浆的含水和氧化中间体的侵位有关,以响应造山地体中地壳的时间和地球化学演化。在这项研究中,综合了中酸性火成岩的全岩和锆石地球化学和同位素数据集,以表征中亚造山带(CAOB)斑岩矿床的地壳演化和成矿。Cu ± Au ± Mo 和 Mo 矿床的成矿斑岩与贫瘠火成岩的 ΔFMQ(氧逸度的代表)较高但大部分重叠。然而,Cu ± Au ± Mo 矿床的成矿斑岩的特征是全岩 V/Sc 和锆石 Eu/Eu*(两者都是含水量的代表)明显高于贫瘠岩石。此外,V/Sc比值与钙碱斑岩铜矿的铜吨位呈正相关,表明岩浆含水量可能对金属禀赋产生一级控制。古生代到中生代 V/Sc 普遍下降,加上 V/Sc 与 K2O 和 SiO2 呈负相关,也表明 CAOB 地壳的逐渐演化明显控制了斑岩型系统的端元(即 Cu ‒ Au 和 Mo 矿床)。c.建议将 Nd 同位素的 500 Ma 作为区分 CAOB 斑岩 Cu ± Au ± Mo (TRes < 500 Ma) 和 Mo 沉积物 (TRes > 500 Ma) 的阈值。锆石 Hf 同位素和地壳厚度的耦合揭示了 CAOB 东部和西部的基本地壳结构分别由晚二叠世和晚石炭世形成,其中 εHf(t) 从大约 250 Ma 开始的趋同趋势突出显示CAOB 东段和西段约 300 Ma,表明碰撞后幼壳的再加工和均质化。在CAOB东部,斑岩型Cu±Au±Mo矿床是由薄岛弧中的幼年物质形成的,而斑岩型Mo矿床是由c.250 Ma以后加厚造山壳中的再加工物质形成的。在西部 CAOB,晚石炭世期间巴尔喀什地区的斑岩铜矿床形成于加厚的大陆地壳(通常大于 40 公里)中,这在遗传上与由加速产生的幼年地壳引发的全球岩浆添加率(MAR)的顶点有关,在与早古生代形成于薄岛弧(一般<40km)中的斑岩型Cu±Au±Mo矿床形成对比。本研究测试锆石 ΔFMQ 作为 fO2 的代表,以及锆石 Eu/Eu* 和全岩 V/Sc 比率作为含水量的代表。说明全岩 V/Sc 比是判断斑岩型 Cu ± Au ± Mo 矿床 Cu 吨位的有利指标,CAOB 独特的斑岩型矿化受地壳成分和厚度反映的地壳演化控制。 .
更新日期:2021-03-05
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