High‐pressure granulite facies re‐equilibration and zoisite–biotite dehydration melting during decompression of an ultrahigh‐pressure garnet clinopyroxenite from the island of Fjørtoft, Norway
High‐pressure granulite facies re‐equilibration and zoisite–biotite dehydration melting during decompression of an ultrahigh‐pressure garnet clinopyroxenite from the island of Fjørtoft, Norway
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DOI：
10.1111/jmg.12649
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发表时间：
2021-12
影响因子：
3.4
通讯作者：
Penglei Liu;H. Massonne
中科院分区：
文献类型：
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作者：
Penglei Liu;H. Massonne
来自挪威西部片麻岩地区菲约托夫特岛的石榴石单斜辉石岩经历了复杂的变质演化，其研究包括岩相学观察、矿物化学、相平衡模拟和地质温压测量。该岩石原为榴辉岩，记录了高于3.5 Gpa的超高压条件(阶段1)，蓝晶石中的富钙石榴石包裹体证明了这一点。超高压组合在1.75 Gpa和870℃左右的高压麻粒岩相条件下普遍重新平衡，在此期间和(或)稍早形成贫钙石榴石、钠辉石、橄榄石和黑云母(阶段2a)。闪锌矿和黑云母的脱水熔融(阶段2b)在相似的压温条件下发生。熔体贫硅富碱，主要结晶为方沸石+斜长石+钾长石。在单斜辉石中发现的石英+角闪石+多硅白云母±石榴石定向复合包裹体被解释为早期退缩阶段3流体(含水熔体和/或水流体)与单斜辉石相互作用的结果。阶段1是岩石深俯冲的结果。第二阶段可能是由于热而加厚的造山带根部的停滞而导致的早期折返。因此，与所研究的石榴石辉石岩相似的、广泛分布于WGR西北部的其他高压麻粒岩相变基性岩也可能在此阶段由原榴辉岩改造而来。此外，该研究还提供了碰撞带中过碱性熔体的产生与地壳深熔作用之间的直接联系，并对高压-超高压矿物中定向矿物包裹体的逆行形成提供了新的见解。
A garnet clinopyroxenite from the island of Fjørtoft, Western Gneiss Region (WGR) in Norway, has experienced a complex metamorphic evolution demonstrated by studies involving petrographic observations, mineral chemistry, phase equilibria modelling and geothermobarometry. This rock, originally an eclogite, documents ultrahigh‐pressure (UHP) conditions higher than 3.5 GPa (Stage 1), witnessed by Ca‐rich garnet inclusions in kyanite. The UHP assemblage was pervasively re‐equilibrated at high‐pressure (HP) granulite facies conditions around 1.75 GPa and 870°C, during which and/or somewhat before Ca‐poor garnet, sodian diopside, zoisite and biotite formed (Stage 2a). Dehydration melting of zoisite and biotite (Stage 2b) took place at similar pressure–temperature conditions. The produced melt was poor in Si and rich in alkalis and crystallized mainly to analcime + plagioclase + K‐feldspar. Oriented composite inclusions of quartz + amphibole + phengite ± garnet, found in clinopyroxene, are interpreted to have resulted from interaction between a fluid (hydrous melt and/or aqueous fluid) and clinopyroxene at the early retrogression Stage 3. Stage 1 resulted from deep subduction of the rock. Stage 2 followed after early exhumation probably due to stagnation at a hot and thickened orogenic root. Consequently, other HP granulite facies metabasites, which are similar to the studied garnet clinopyroxenite and widespread in the northwestern WGR, could have been transformed from former eclogite at this stage, too. In addition, this study provides a direct link between the generation of peralkaline melts and crustal anatexis in collision zones and new insights into the retrograde formation of oriented mineral inclusions in HP‐UHP minerals.