Petrogenesis of leucosome sheets inmigmatitic UHP eclogites—Evolution from silicate-rich supercritical fluid to hydrous melt
Petrogenesis of leucosome sheets inmigmatitic UHP eclogites—Evolution from silicate-rich supercritical fluid to hydrous melt
复制标题
混合岩超高压榴辉岩中无色体片的岩石成因——从富含硅酸盐的超临界流体到含水熔体的演化
DOI：
10.1016/j.lithos.2020.105442
复制
发表时间：
2020
期刊：
影响因子：
3.5
通讯作者：
Zhenlong Wang
中科院分区：
文献类型：
--
作者：
Songjie Wang;Lu Wang;Michael Brown;Tim Johnson;Philip Piccoli;Peng Feng;Zhenlong Wang
最近对中国苏鲁带中部超高压带榴辉岩的研究表明，在从变质峰折返到高压榴辉岩相条件的过程中，白硅白云岩保持稳定。这一观察结果要求在超高压变质条件下折返期间产生熔体的过程中涉及一些其他流体来源。在这里，我们研究了苏禄带中部退行和混合岩超高压榴辉岩中的分米到米厚的无色体片。无色体具有可变的矿物模式和变形结构；有些是由边缘相和内部相复合而成，而另一些则不是。它们的范围从花岗岩到长辉石，具有高丰度的 SiO2、K2O + Na2O 和 Al2O3，与含水熔体类似，并且具有与白白云母和钠长石 + 绿帘石感观的模态变化一致的主要氧化物和微量元素变化。无色体相对于高场强元素富集大离子亲石元素，并且具有相对于重稀土元素富集轻稀土元素的稀土元素（REE）模式，通常具有小的正或负Eu异常。无色体中新结晶的锆石记录了加权平均年龄ofc。 223–218 马。早期形成的隐色体的 Sr-Nd 同位素组成与附近未退缩的超高压榴辉岩相似，而较年轻的隐色体的 Sr-Nd 同位素组成介于榴辉岩和周围片麻岩之间。钛锆石测温法与硅硅白云母气压计相结合，表明无色体在约 850 至 770 °C 之间、压力 (P) 范围为 3.5 至 2.2 GPa 之间发生结晶，这与从年长（较高 P）到年轻（较低 P）的年龄相关。在变质峰值（可能超过 5.5 GPa）时，烃源岩可能缺乏流体或不存在流体。在超高压条件下的折返过程中，我们假设储存在名义上无水矿物中的水的出溶在榴辉岩中形成了富含硅酸盐的超临界流体，并演变成更致密、更粘稠和更聚合的含水熔体。多硅白云石气压分析表明，早期形成的隐色体在接近玄武岩加水系统临界线的压力下结晶，可能是由于水扩散到宿主榴辉岩中而导致的。富含溶质的超临界流体从周围片麻岩中渗透，与榴辉岩中的熔体混合，在这些端元之间产生可变的 Sr-Nd 同位素组成中间体，如在较低压力下结晶的隐色体所记录的那样。最年轻的隐色体在接近钙花岗岩加水系统临界线的压力下结晶，产生少量的水性流体，沉淀出与隐色体片相关的石英脉，特别是在其边缘。随后，白白云石中有限的白白云石分解熔化被白白云石周围的斜长石+黑云母的聚集体以及沿着晶界的薄膜和尖状细脉和钾长石斑块记录下来。相平衡模型表明，这种晚期熔融发生在接近从高压榴辉岩到角闪岩相过渡的 P-T 条件下，最终的亚固相线平衡为 1.0-0.9 GPa 和 T< 640 °C。
Recent studies of eclogite in the ultrahigh pressure (UHP) zone of the central Sulu belt in China have shown that phengite remained stable during exhumation from the metamorphic peak to HP eclogite facies conditions. This observation requires that some other source of fluid was involved in the production of melts generated during exhumation from UHP metamorphic conditions. Here, we investigate decimeter- to meter-thick leucosome sheets in retrogressed and migmatitic UHP eclogite in the central Sulu belt. The leucosomes have variable mineral modes and deformation fabrics; some are composite with marginal and interior facies, whereas others are not. They range from granite to trondhjemite, with high abundances of SiO2, K2O + Na2O and Al2O3, similar to hydrous melts, and have major oxide and trace element variations consistent with modal changes in phengite and albite + epidote sensulato. The leucosomes are enriched in large ion lithophile elements relative to high field strength elements and have rare earth element (REE) patterns enriched in light REE relative to heavy REE, generally with small positive or negative Eu anomalies. Newly-crystallized zircon in the leucosomes records weighted mean ages ofc. 223–218 Ma. Early-formed leucosome has Sr-Nd isotope compositions similar to those of nearby unretrogressed UHP eclogites, whereas younger leucosome has Sr-Nd isotope compositions intermediate between the eclogites and surrounding gneisses. Ti-in-zircon thermometry combined with Si-in-phengite barometry indicates crystallization of the leucosomes between ~850 and ~770 °C, over a wide range of pressure (P) from 3.5 to 2.2 GPa, which correlates with age from older (higherP) to younger (lowerP). At the metamorphic peak, which may have exceeded 5.5 GPa, the source rocks were likely fluid deficient or fluid absent. During exhumation from UHP conditions, we posit that exsolution of water stored in nominally anhydrous minerals formed a silicate-rich supercritical fluid in eclogite that evolved to a denser, more viscous and more polymerized hydrous melt. Phengite barometry indicates that the early-formed leucosomes crystallized at pressures close to the critical line for the basalt plus water system, possibly by diffusive loss of water to the host eclogite. Infiltration of solute-rich supercritical fluid from the surrounding gneisses blended with melt in the eclogites generating variable Sr-Nd isotope composition intermediate between these end-members, as recorded by leucosomes that crystallized at lower pressures. The youngest leucosomes crystallized at pressures near the critical line for the Ca-granite plus water system creating a low volume of aqueous fluid that precipitated quartz veins associated with the leucosome sheets, particularly at their margins. Subsequently, limited phengite-breakdown melting in the leucosomes is recorded by aggregates of plagioclase + biotite around phengite and thin films and cuspate veinlets and patches of K-feldspar along grain boundaries. Phase equilibrium modelling indicates that this late stage melting occurred atP–Tconditions near the transition from HP eclogite to amphibolite facies, with final subsolidus equilibration at 1.0–0.9 GPa andT< 640 °C.