Compositional zoning in dolomite from lawsonite-bearing eclogite (SW Tianshan, China): Evidence for prograde metamorphism during subduction of oceanic crust
Compositional zoning in dolomite from lawsonite-bearing eclogite (SW Tianshan, China): Evidence for prograde metamorphism during subduction of oceanic crust
复制标题
DOI：
10.2138/am.2014.4507
复制
发表时间：
2014
影响因子：
3.1
通讯作者：
Ji‐Lei Li;R. Klemd;Jun Gao;M. Meyer
中科院分区：
文献类型：
--
作者：
Ji‐Lei Li;R. Klemd;Jun Gao;M. Meyer
摘要在中国西北天山（超）高压低温变质带的碳酸盐-硬铝石榴辉岩中发现了具有成分分带的白云岩。榴辉岩相白云岩以基质变斑晶和石榴子石中的包裹体形式产出，二者具有相同的化学分带模式。白云石中含有方解石（可能在文石之后）、菱镁矿、蓝闪石、硬铝石（及其假晶）、褐帘石、绿帘石、钠云母、多硅白云母和绿辉石。白云岩的化学分带是由中心到边缘的连续Mg增加和Fe-Mn减少来定义的。过渡金属元素、REE和Y的浓度也从白云石的核心到边缘逐渐降低。热力学模拟表明，白云岩的Fe-Mg分带在很大程度上取决于温度，因此可以解释为碳酸盐洋壳俯冲过程中形成的逆冲生长分带。据认为，白云石与石榴石平衡形成的结果，由于温度升高而改变基质成分。此外，热力学模拟表明，在高压条件下的俯冲过程中，变形形成的文石和白云石转化为白云石和菱镁矿。此外，富铁的菱镁矿包裹体在基质白云岩和白云岩包裹体在石榴石已形成在高压条件之前的变质条件，因此，谨慎使用含铁的菱镁矿出现在榴辉岩相岩石作为一个明确的压力指标，如前所述。
Abstract Dolomite with compositional zoning was discovered in carbonate-lawsonite-bearing eclogites in the Tianshan (ultra-)high-pressure/low-temperature metamorphic belt, northwestern China. The eclogitefacies dolomite occurs as matrix porphyroblast and as inclusion in garnet, both of which display the same chemical zoning pattern. The dolomite contains inclusions of calcite (probably after aragonite), magnesite, glaucophane, lawsonite (and its pseudomorphs), allanite, epidote, paragonite, phengite, and omphacite. The chemical zoning in dolomite is well defined by a continuous core-to-rim Mg increase and Fe-Mn decrease. The concentrations of transition metal elements, REE, and Y also decrease from core to rim of the dolomite. Thermodynamic modeling demonstrates that the Fe-Mg zoning of dolomite is largely temperature dependent and, thus, is interpreted as prograde growth zoning, which developed during subduction of carbonate-bearing oceanic crust. It is suggested that dolomite in equilibrium with garnet formed as a result of changing matrix compositions due to increasing temperatures. In addition, thermodynamic modeling demonstrates that during subduction at high-pressure conditions prograde-formed aragonite and dolomite were transformed to dolomite and magnesite. Furthermore, Fe-rich magnesite inclusions in matrix dolomite and in dolomite inclusions in garnet are shown to have formed during high-pressure conditions prior to peak metamorphic conditions and, therefore caution is warranted using Fe-bearing magnesite occurrences in eclogite-facies rocks as an unambiguous ultrahigh pressure indicator as previously suggested.