榴辉岩韧性剪切变形中的流体作用及地球化学效应

韧性剪切带是在较高温条件下机械与化学作用高度耦合的系统。韧性剪切带普遍含有一种或多种流体,流体是韧性剪切变形高度局域化和剪切变形组构形成的主要影响因素。在强应变岩石中,可利用变形矿物的元素地球化学的变化特征来反演参与变形的流体的地球化学性质。苏鲁-大别超高压变质带中,多数榴辉岩经历了强烈的韧性剪切应变和流体作用,形成高度局域化、尺度不等的韧性剪切变形带,为研究在高压-超高压条件下,参与流体的性质,应变和流体相互作用提供了重要机会。样品DH-2G是一强烈剪切榴辉岩,由石榴石、绿辉石、金红石,及少量的多硅白云母、黝帘石、角闪石、斜长石、石英、锆石和磷灰石组成。在不同应变域,矿物的粒度变化明显。在类变斑晶域,石榴石粒度比剪切条带中的小2~4倍。在以下讨论中,与细粒石榴石(石榴石-I)共生的绿辉石称为绿辉石-I,与粗粒石榴石(石榴石-II)共生的绿辉石称为绿辉石-II。电子探针和原位LA-ICP-MS测试结果表明:(1)除Ni外,两类石榴石没有显著区别;(2)两类绿辉石在主量元素上也没有明显的差异,但微量元素特征,尤其是稀土元素上,存在明显的变化。与绿辉石-I相比,绿辉石-II具有以下特征:(1)轻稀土明显降低,达25~40倍左右;(2)重稀土明显富集,升高5~10倍;(3)Ni、Zn、Co、Sc、Ba、Zr、Y、Rb、Hf和HREE明显升高,而 V、Sr、U、Th、Pb和LREE明显降低;和(4)微弱的正Eu异常, Eu/Eu^*=1.04~1.45,绿辉石-I基本上没有Eu异常。并且,剪切带域金红石的Nb/Ta比值从核部(<19.0)到边部(>20.0)显著增加。岩相学、元素和温压估计研究表明:在剪切过程中不存在外来流体,流体是由无水矿物(绿辉石、石榴石、金红石等)在超高压榴辉岩折返减压过程中通过压力降低出溶结构羟基或分子水,释放出流体,流体富含V、Sr、U、Th、Pb、Zr、Nb和LREE等微量元素。这些流体显著影响深俯冲地壳的流变学性质,是迁移活动元素(LILE和LREE)及保守元素(HFSE等)的重要载体,对超高压岩片的快速折返发挥关键作用。

Nature and geochemical effects of fluids during ductile shearing of UHP eclogite.

Ductile shear zone is a highly coupled system that involves both mechanical and chemical processes at the presence of aqueous fluids at relatively high temperature conditions. Presence of fluids is one of the major factors that lead to highly localized ductile deformation and the formation of shear fabric. Therefore, variations in mineral chemistry in highly strained rocks can be used to infer the geochemical nature of fluids involving in ductile deformation. Sample DH-2G, an intensively sheared eclogite, consists of garnet, omphacite, rutile, and minor phengite, zoisite, hornblende, plagioclase, quartz, zircon and apatite. There are great variations in garnet grain sizes across different strain domains. In porphyroblast-like domain, garnet grains are only half to one-fourth of the sizes of those within the sheared band. Omphacite and rutile show similar pattern. Electron microprobe and LA-ICP-MS analyses were performed on garnet, omphacite, and rutile grains. These data are used to discuss how shearing strain affects the trace element geochemistry of garnet, omphacite, and rutile. In the following discussion, we call those omphacite grains coexisting with fine garnet (garnet-I) as omphacite-I, and those with coarse garnet (garnet-II) as omphacite-II. Our data show that (1) major and trace element contents in garnet-I and garnet-II are almost the same, except Ni; and (2) omphacite-I and omphacite-II also do not show substantial differences in major element composition; but they do have significant differences in trace element geochemistry, especially in REEs. As compared to those in omphacite-I, omphacite-II have (1) lower LREE by up to 25~40 times; (2) elevated HREE by up to 5~10 times; (3) higher contents of Ni, Zn, Co, Sc, Ba, Zr, Y, Rb, Hf, but lower V, Sr, U, Th, Pb, and (4) weak to strong positive Eu anomalies with Eu/Eu^*=1.0~1.5; Furthermore, Nb/Ta ratio values of rutiles in sheared domains dramatically increase from the core toward the rim from <19.0 to >20.0. Petrography, element geochemistry, and P-T estimations indicate that fluids involved in the ductile shearing possiblby were internally buffered, and are highly enriched in V, Sr, U, Th, Pb, Zr, Nb and LREE. These fluids might derived from decompressional dehydration of normally anhydrous minerals (e.g. omphacite and rutile). The fluids, the important carrier of mobile elements (LILE and LREE) and relatively immobile elements (HFSE), conspicuously affect the rheology of subduction crust and could contribute to the rapid exhumation of UHP slab.