Garnet-forming reactions in felsic orthogneiss: Implications for densification and strengthening of the lower continental crust
Garnet-forming reactions in felsic orthogneiss: Implications for densification and strengthening of the lower continental crust
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长英质正片麻岩中的石榴石形成反应：对下大陆地壳致密化和强化的影响
DOI：
10.1016/j.epsl.2014.08.030
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发表时间：
2014
影响因子：
5.3
通讯作者：
Holland, M.
中科院分区：
文献类型：
--
作者：
Williams, M.L.;Dumond, G.;Mahan, K.;Regan, S.;Holland, M.
萨斯喀彻温省北方阿萨巴斯卡麻粒岩带（AGT）正片麻岩中石榴石和辉石的生长为下陆壳的逐步致密化和强化提供了一个模式，并对陆壳的再循环和长期演化产生了影响。花岗质和花岗闪长质片麻岩中保存有两种不同的组合和结构。低应变正片麻岩具火成结构，组合为Opx+ Kfs+ Pl+ Mag+ Qz（±Bt，Hbl，Ilm）。高应变，动态重结晶构造岩有额外的石榴石，单斜辉石，和更富钠斜长石，沿着与残余斜方辉石。这种反应（Opx+富Ca Pl= Grt+ Cpx+富Na Pl+ Qz）在记录了玛丽花岗岩基中的事件后被非正式地称为“玛丽反应”。该反应代表了从中压到高压麻粒岩的转变（绿色和Ringwood，1967），但在这些深部地壳岩石中，反应过程是沿着一条等压冷却路径进行的，温度约为200 ℃。1 GPa（35-40 km深度）。环境P-T条件完全在产品（低温侧）稳定性范围内。产物组合（Grt+ Cpx+富Na Pl）的丰度随变形程度的增加而增加。亚稳火成岩组合广泛保存在低应变样品中。随着应变的增加，石榴石出现在斜长石碎斑岩的重结晶地幔中，单斜辉石出现在斜方辉石晶体的变形尾部。变形被解释为有助于斜长石的分解和/或石榴石和单斜辉石的成核。石榴石和辉石模式已被观察到超过10体积%的AGT，但更大的数量是可能的，因为富含钙的斜长石和斜方辉石残留物被广泛保存。密度从CA增加。2.6至约3.0 g/cm 3，长英质岩石P波速度接近7.0km/s。镁铁质岩石的密度接近3.4立方厘米。该反应在AGT（2.6和1.9 Ga）中至少发生过两次，并且可能在长期地壳深部驻留期间的其他时间发生过。因此，致密化可以递增地发生，并且与构造作用和伴随的重结晶的时间和强度相对应。这种反应提供了一种机制，使下陆壳在初始稳定化后很长时间内显著致密化和强化。这可以解释在地壳较低的深度和暴露的麻粒岩岩石中所观察到的长英质岩石丰度的地震速度之间的差异暗示镁铁质/中间成分。
Growth of garnet and pyroxene in orthogneiss from the Athabasca granulite terrane (AGT), northern Saskatchewan, provides a model for progressive densification and strengthening of the lower continental crust with implications for the recycling and long-term evolution of continental crust. Two distinct assemblages and textures are preserved in granitic and granodioritic gneiss. Low-strain orthogneiss displays igneous textures and assemblages of Opx+ Kfs+ Pl+ Mag+ Qz (±Bt, Hbl, Ilm). High-strain, dynamically recrystallized tectonites have additional garnet, clinopyroxene, and a more Na-rich plagioclase, along with relict orthopyroxene. The reaction (Opx+ Ca-rich Pl= Grt+ Cpx+ Na-rich Pl+ Qz) is informally called the “Mary reaction” after documented occurrences in the Mary granitoid batholith. The reaction represents the transition from medium-pressure to high-pressure granulite (Green and Ringwood, 1967), but reaction progress was achieved in these deep crustal rocks along an isobaric cooling path at ca. 1 GPa (35–40 km-depth). Ambient P–T conditions were well within the product (low-T-side) stability field. The abundance of the product assemblage (Grt+ Cpx+ Na-rich Pl) increases with deformation. Metastable igneous assemblages are widely preserved in low-strain samples. With increasing strain, garnet occurs within recrystallized mantles of plagioclase porphyroclasts, and clinopyroxene occurs in the deformed tails of orthopyroxene crystals. Deformation is interpreted to aid in the breakdown of plagioclase and/or the nucleation of garnet and clinopyroxene. Garnet and pyroxene modes have been observed to exceed 10 vol% in the AGT, but larger amounts are possible because Ca-rich plagioclase and orthopyroxene remnants are widely preserved. Densities increase from ca. 2.6 to ca. 3.0 g/cm 3 and modeled P-wave velocities approach 7.0 km/s in felsic rocks. Densities in mafic rocks approach 3.4 cm 3. The reaction occurred at least twice in the AGT, 2.6 and 1.9 Ga, and may have occurred at other times during long-term deep crustal residence. Thus, densification can occur incrementally and correspond with the time and intensity of tectonism and concomitant recrystallization. The reaction provides a mechanism for significant densification and strengthening of lower continental crust long after initial stabilization. It may explain the discrepancy between seismic velocities suggestive of mafic/intermediate compositions at lower crustal depths and the observed abundance of felsic rocks in exposed granulite terranes.
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