Growth of hydrothermal baddeleyite and zircon in different stages of skarnization
Growth of hydrothermal baddeleyite and zircon in different stages of skarnization
复制标题
DOI：
10.2138/am-2016-5706
复制
发表时间：
2016-12
影响因子：
3.1
通讯作者：
W. Zhao;Mei‐Fu Zhou;W. Chen
中科院分区：
文献类型：
--
作者：
W. Zhao;Mei‐Fu Zhou;W. Chen
滕铁铁存款矿床中的逆冲和退冲矽卡岩均含有圆形、自形和他形锆石颗粒。圆形颗粒最初来自碳酸盐岩中的碎屑，并被并入矽卡岩中。自形和他形晶体与各种矽卡岩矿物共生，显然是热液成因。这些热液颗粒具有较低的（Sm/La）N比值和较高的La含量相对于典型的岩浆，并显示脂肪LREE和抑制肥化的HREE磷灰石归一化模式，类似于那些从Zr饱和流体结晶的锆石。前级矽卡岩中还含有锆石镶边的斜锆石，相对于原始的岩浆热液流体，这记录了一个低硅活动期。腾铁热液锆石具有变化的Eu异常和轻微的正Ce异常，表明它们可能是从高度不均匀但普遍还原的流体中结晶出来的。它们的δ 18 O值较低（-5.1 ~-2.7 ‰），表明有大气流体的参与。富氟流体在高场强元素（包括Zr）从寄主花岗岩向矽卡岩系统迁移和迁移过程中起着重要作用。含HFSE的流体与碳酸盐岩在超高压阶段发生反应，将F络合物分解为富含HFSE的矽卡岩矿物和斜锆石存款。在退变质阶段，富含HFSE的矽卡岩矿物的蚀变释放了HFSE，包括Zr和Sn，从而产生了锆石、镁黄长石和退变质矽卡岩矿物的矿物组合。对腾铁夕卡岩系中锆石的西姆斯测年结果表明，夕卡岩化作用的持续时间大约在几百万年以内。研究表明，Zr不仅在有利条件下具有局部移动的性，而且在矽卡岩化作用的不同阶段也易于迁移和沉积。
Abstract Both prograde and retrograde skarns from the Tengtie iron deposit, South China, contain rounded, euhedral, and anhedral zircon grains. Rounded grains were originally derived from detritus in carbonate rocks and were incorporated into the skarns. Euhedral and anhedral crystals are intergrown with various skarn minerals and are clearly hydrothermal in origin. These hydrothermal grains have low (Sm/La)N ratios and high La contents relative to typical magmatic ones and display fat LREE and subdued fattening of HREE chondrite-normalized patterns, similar to those of zircon crystallized from Zr-saturated fluids. Prograde skarns also contain baddeleyite rimed by zircon, which record a period of low Si activity during prograde skarnization relative to original magmatic-hydrothermal fluids. Hydrothermal zircon grains from Tengtie have variable Eu anomalies and slightly positive Ce anomalies, indicating that they may have crystallized from highly heterogeneous, but generally reducing fluids. They have low δ18O values (–5.1 to –2.7 ‰), suggesting the involvement of meteoric fluids. Fluorine-rich fluids played an important role in remobilizing and transporting some high field strength elements (HFSE), including Zr, from the host granites into the skarn system. Reaction between HFSE-bearing fluids and carbonate rocks at the prograde stage decomposed F complexes to deposit HFSE-rich skarn minerals and baddeleyite. At the retrograde stage, alteration of the HFSE-rich skarn minerals released HFSE, including Zr and Sn, consequently producing a mineral assemblage of zircon, cassiterite, and retrograde skarn minerals. Dating results of zircons from the Tengtie skarn system by SIMS indicates roughly less than several million years duration for skarnization. Our study indicates that Zr was not only mobile locally under favorable conditions, but was also readily transported and deposited in different stages of skarnization.