Low-P and high-T metamorphism of basalts: Insights from the Sudbury impact melt sheet aureole and thermodynamic modelling
Low-P and high-T metamorphism of basalts: Insights from the Sudbury impact melt sheet aureole and thermodynamic modelling
复制标题
DOI：
10.1111/jmg.12460
复制
发表时间：
2019-02-01
影响因子：
3.4
通讯作者：
Lesher, C. Michael
中科院分区：
文献类型：
--
作者：
Jorgensen, Taus R. C.;Tinkham, Douglas K.;Lesher, C. Michael
玄武质岩石的低压高温（LP-HT）变质作用在全球范围内发生，贯穿整个地质时期，很少受到正演相平衡模型的约束，但这种计算提供了有价值的补充测温信息和对深熔作用的约束，而这是不可能从常规测温中获得的。沿着萨德伯里火成杂岩（SIC）南缘的变质玄武岩记录了涉及部分熔融和熔融分离的高级接触变质作用的证据。峰值变质温度达到至少类似于925摄氏度，在类似于1-3千巴附近的SIC接触。保存的峰值矿物组合表明，大多数产生的熔体逃离这些岩石留下的残余物，其特征在于斜长石-斜方辉石-单斜辉石-钛铁矿-磁铁矿+/-熔体组合。从SIC下部接触点到500米处，峰值温度达到了875摄氏度，这标志着向只经历了初期部分熔融而没有熔融损失的变玄武岩的过渡。距离SIC接触面约500 ~ 750 m的变质玄武岩具有相似的二辉石矿物组合特征，但通常含有丰富的角闪石，沿等压冷却路径沿着长出斜方辉石和斜方辉石。000 m处的角闪石-斜长石-石英-钛铁矿组合表明温度高达680摄氏度。质量平衡和相平衡计算表明，深熔导致10-20%的熔融产生的内部类似于500米的光环，甚至更高程度的熔融走向接触。多个模型，实验和天然样品的比较表明，在Na 2 O-CaO-FeO-MgO-Al 2 O3-SiO2-H2O-TiO 2-O-2（NCFMASHTO）系统中建模的结果是最可靠的预测固相线的温度。在最近的角闪石溶液模型中加入K2 O，现在成功地预测了高钙角闪石和硅酸盐熔体在相对较低的压力（类似于1.5千巴）下共存的脱水熔融。然而，包含K2 O作为系统组分导致在过低的温度下预测固相线。虽然有建模预测和实验结果之间的差异，这项研究表明，镁铁质岩石的伪截面方法是一个宝贵的工具，以限制在LP-HT条件下的变质过程。
Low-pressure and high-temperature (LP-HT) metamorphism of basaltic rocks, which occurs globally and throughout geological time, is rarely constrained by forward phase equilibrium modelling, yet such calculations provide valuable supplementary thermometric information and constraints on anatexis that are not possible to obtain from conventional thermometry. Metabasalts along the southern margin of the Sudbury Igneous Complex (SIC) record evidence of high-grade contact metamorphism involving partial melting and melt segregation. Peak metamorphic temperatures reached at least similar to 925 degrees C at similar to 1-3 kbar near the SIC contact. Preservation of the peak mineral assemblage indicates that most of the generated melt escaped from these rocks leaving a residuum characterized by a plagioclase-orthopyroxene-clinopyroxene-ilmenite-magnetite +/- melt assemblage. Peak temperatures reached similar to 875 degrees C up to 500 m from the SIC lower contact, which marks the transition to metabasalts that only experienced incipient partial melting without melt loss. Metabasalts similar to 500 to 750 m from the SIC contact are characterized by a similar two-pyroxene mineral assemblage, but typically contain abundant hornblende that overgrew clino- and orthopyroxene along an isobaric cooling path. Metabasalts similar to 750 to 1,000 m from the SIC contact are characterized by a hornblende-plagioclase-quartz-ilmenite assemblage indicating temperatures up to similar to 680 degrees C. Mass balance and phase equilibria calculations indicate that anatexis resulted in 10-20% melt generation in the inner similar to 500 m of the aureole, with even higher degrees of melting towards the contact. Comparison of multiple models, experiments, and natural samples indicates that modelling in the Na2O-CaO-FeO-MgO-Al2O3-SiO2-H2O-TiO2-O-2 (NCFMASHTO) system results in the most reliable predictions for the temperature of the solidus. Incorporation of K2O in the most recent amphibole solution model now successfully predicts dehydration melting by the coexistence of high-Ca amphibole and silicate melt at relatively low pressures (similar to 1.5 kbar). However, inclusion of K2O as a system component results in prediction of the solidus at too low a temperature. Although there are discrepancies between modelling predictions and experimental results, this study demonstrates that the pseudosection approach to mafic rocks is an invaluable tool to constrain metamorphic processes at LP-HT conditions.