Multi-stage infiltration of Na- and K-rich fluids from pegmatites at mid-crustal depths as revealed by feldspar replacement textures
Multi-stage infiltration of Na- and K-rich fluids from pegmatites at mid-crustal depths as revealed by feldspar replacement textures
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长石置换结构揭示了中地壳深处伟晶岩中富钠和富钾流体的多阶段渗透
DOI：
10.1016/j.lithos.2021.106096
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发表时间：
2021
期刊：
影响因子：
3.5
通讯作者：
Tsuchiya Noriyoshi
中科院分区：
文献类型：
--
作者：
Nurdiana Astin;Okamoto Atsushi;Yoshida Kenta;Uno Masaoki;Nagaya Takayoshi;Tsuchiya Noriyoshi
在火山区，地球物理观测发现中地壳侵入物周围存在深层流体。然而，人们对地壳高温下的流体组成和流体运移机制仍然知之甚少。在这项研究中，我们研究了日本东北部金华山岛与石英闪长岩侵入体接触处的镁铁质片岩（单斜辉石 [Cpx] 片岩、角闪石 [Hbl] 片岩和阳起石 [Act] 片岩）中的斜长石蚀变。石英闪长岩由角闪石和斜长石组成，在 670–760 °C 和 0.30–0.45 GPa 下结晶。石英闪长岩含有丰富的伟晶岩脉，其边缘由斜长石组成，中心由钾长石+石英±石榴石组成。一些伟晶岩脉切割了变质岩的片岩。在变质岩中，在冷却过程中发现了两个由流体介导的蚀变阶段。流体渗透的第一阶段（第一阶段）的特征是在 Cpx 片岩中伟晶岩脉周围形成反应区。 Cpx 更改为角闪石 (Hbl)，Ca-斜长石 (An83-95) 更改为 Na-斜长石 (An36-67)。置换沿着斜长石和单斜辉石的晶界进行。第二阶段（Stage II）导致Act和Hbl片岩中钾长石矿脉的形成，斜长石（An35-57）被钾长石（An0Ab1Or99）和钠长石（An2Ab95Or3-An12Ab87Or1）取代，后者可能源自Cpx片岩。假设压力与石英闪长岩侵入体的结晶相同，则第一阶段的蚀变温度估计为 690–730 °C，第二阶段的蚀变温度为 400–570 °C。在斜长石和单斜辉石/角闪石颗粒替换过程中，考虑体积守恒的质量平衡考虑表明，Cpx 片岩在阶段 I 中获得了 Na 和 H2O，并失去了 Ca，而 Act 片岩在阶段 II 中，在 Na 几乎恒定的情况下获得了 K 和 Si，并失去了 Al 和 Ca。这表明冷却过程中流体成分从富钠变为富钾。在第二阶段蚀变过程中，当~45%的斜长石颗粒发生蚀变时，斜长石中产生了显着的纳米至微米级孔隙，导致斜长石孔隙度达到~3.0%~3.5%，全岩孔隙度增加了1.3%±0.2%。这些孔隙现在是孤立的，形状不规则，并且优先沿着置换前沿出现，表明孔隙在置换期间或之后发生了迁移。斜长石中的蚀变区是由穿粒微裂纹而不是晶界形成的。我们的结果表明，富钾流体的渗透可能是自我促进的，这些流体在斜长石替换过程中通过溶解和沉淀过程形成自己的路径。鉴于斜长石是中地壳各种岩石类型中的主要矿物，斜长石中反应引起的孔隙网络可能是中地壳深度的主要流体通道。这种孔隙率和流体迁移的性质可能由温度和/或流体的成分控制。
In volcanic zones, geophysical observations have identified the presence of deep-seated fluids around intrusions in the mid-crust. However, the fluid compositions and mechanisms of fluid migration at high temperatures in the crust are still poorly understood. In this study, we investigated plagioclase alteration in mafic schists (clinopyroxene [Cpx] schist, hornblende [Hbl] schist, and actinolite [Act] schist) at the contact with a quartz diorite intrusion on Kinkasan Island, NE Japan. The quartz diorite consists of hornblende and plagioclase, and crystallized at 670–760 °C and 0.30–0.45 GPa. The quartz diorite contains abundant pegmatitic veins, which consist of plagioclase at their margins and K-feldspar + quartz ± garnet in their centers. Some pegmatitic veins cut the schistosity of the metamorphic rocks. In the metamorphic rocks, two stages of fluid-mediated alteration during cooling were recognized. The first stage of fluid infiltration (Stage I) was characterized by the formation of a reaction zone around pegmatitic dikes in the Cpx schists. The Cpx was altered to hornblende (Hbl) and Ca-plagioclase (An83–95) was altered to Na-plagioclase (An36–67). The replacement proceeded along the grain boundaries of plagioclase and clinopyroxene. The second stage (Stage II) resulted in the formation of K-feldspar veins and replacement of plagioclase (An35–57) by K-feldspar (An0Ab1Or99) and albite (An2Ab95Or3–An12Ab87Or1) in the Act and Hbl schists, and the latter were probably derived from the Cpx schists. Assuming the pressure was the same as the crystallization of the quartz diorite intrusion, the alteration temperatures were estimated as 690–730 °C for Stage I and 400–570 °C for Stage II. Mass balance considerations with assumptions of the volume conservation during the replacements of plagioclase and clinopyroxene/amphibole grains indicate the Cpx schists gained Na and H2O and lost Ca in Stage I, and the Act schists gained K and Si and lost Al and Ca at almost constant Na in Stage II. This indicates that the fluid composition changed from Na- to K-rich during cooling. During the Stage II alteration, when ~45% of the plagioclase grains were altered, significant nano- to micro-scale pores were generated in plagioclase, which produced a porosity of ~3.0%–3.5% in the plagioclase and increased the whole-rock porosity by 1.3% ± 0.2%. These pores are now isolated, irregularly shaped, and occur preferentially along the replacement front, suggesting that the pores migrated during or after replacement. The altered zone in plagioclase developed from trans-granular microcracks rather than grain boundaries. Our results suggest that infiltration of K-rich fluids could be self-promoting, with such fluids forming their own pathways via dissolution and precipitation processes during plagioclase replacement. Given that plagioclase is the dominant mineral in various rock types in the mid-crust, reaction-induced porosity networks in plagioclase could be the dominant fluid pathway at mid-crustal depths. The nature of this porosity and fluid migration are potentially controlled by temperature and/or the composition of the fluids.