Controls on development of different mineral assemblages in gabbro and basalt during subduction metamorphism
Controls on development of different mineral assemblages in gabbro and basalt during subduction metamorphism
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DOI：
10.1007/s00410-020-01753-6
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发表时间：
2020-11
影响因子：
3.5
通讯作者：
P. Kang;K. Fornash;D. Whitney
中科院分区：
文献类型：
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作者：
P. Kang;K. Fornash;D. Whitney
在高压（P）/低温（T）复杂（Sivrihisar，土耳其）共存的细粒（变火山）和粗粒（变深成）镁铁质岩石保存不同的逆冲，峰值和逆行矿物组合，提供了一个机会，以评估控制矿物组合在经历了相同的P-T条件的变基性岩。细粒变质玄武岩是含石榴子石的硬硅钙石蓝片岩和榴辉岩，具有相似的组合，在毫米至厘米尺度上以蓝闪石与绿辉石的模式变化。相比之下，变辉长岩由不平衡矿物套件组成，该矿物套件由残余火成单斜辉石组成，部分被绿辉石或含水相（硬硅钙石+透闪石或蓝闪石）取代，基质由细粒硬硅钙石、绿辉石、透闪石、白色云母、非常罕见的石榴石和退变矿物（例如，绿帘石、钠长石和钛铁矿），后来有方解石和方解石。假截面模型预测变玄武岩的富蓝闪石（蓝片岩）和富绿辉石（榴辉岩）层的峰值P-T条件（490-530 °C，1.8-2.0 GPa）相似，变辉长岩的峰值P-T条件（490-600 °C，1.9-2.5 GPa）略高。由于变辉长岩中含水矿物的模态丰度较高，因此在变玄武岩（2.0- 5.4wt%）中峰值P-T条件下模拟的H2O含量范围明显低于变辉长岩（6.4- 8.7wt%）。在相对相似的P-T峰值条件下，变辉长岩经历了与共存的变玄武岩不同的反应历史，从而形成了独特的HP/LT矿物组合和模式（例如，稀有的石榴石），这是由于其更富Mg的本体组成（XMg= 0.58-0.84对0.50）、更高的H2O含量和更粗的粒度。该研究首次对Sivrihisar变辉长岩进行了岩石学分析，首次对该地区变基性岩中的H2O含量进行了系统研究，是世界上保存最完好的硬毛石榴辉岩和蓝片岩之一。
Coexisting fine-grained (meta-volcanic) and coarse-grained (meta-plutonic) mafic rocks in a high-pressure (P)/low-temperature (T) complex (Sivrihisar, Turkey) preserve different prograde, peak, and retrograde mineral assemblages, providing an opportunity to evaluate controls on mineral assemblages in metabasites that experienced the same P–T conditions. Fine-grained metabasalts are garnet-bearing lawsonite blueschist and eclogite with similar assemblages that vary on a mm- to cm- scale in mode of glaucophane vs. omphacite. In contrast, metagabbro consists of a disequilibrium mineral suite of relict igneous clinopyroxene partially replaced by omphacite or hydrous phases (lawsonite + tremolite or glaucophane) in a matrix of fine-grained lawsonite, omphacite, tremolite, white mica, very rare garnet, and retrograde minerals (e.g., epidote, albite, and titanite), with later chlorite and calcite. Pseudosection modeling predicts similar peak P–T conditions (490–530 °C, 1.8–2.0 GPa) for both glaucophane-rich (blueschist) and omphacite-rich (eclogite) layers of the metabasalt and similar to slightly higher conditions (490–600 °C, 1.9–2.5 GPa) for metagabbro. The range of modelled H2O content at peak P–T conditions in metabasalt (2.0–5.4 wt%) is significantly lower than in metagabbro (6.4–8.7 wt%) due to the higher modal abundance of hydrous minerals in the latter. At the relatively similar peak P–T conditions, metagabbro experienced different reaction histories from coexisting metabasalt, thereby developing distinctive HP/LT mineral assemblages and modes (e.g., scarce garnet) owing to its more Mg-rich bulk composition (XMg= 0.58–0.84 vs. 0.50), higher H2O content, and coarser grain-size. This study is the first petrologic analysis of Sivrihisar metagabbro and the first systematic study of H2O content in metabasites from this locality, which is one of the best-preserved examples of lawsonite eclogite and blueschist in the world.