A new perspective on metamorphism and metamorphic belts
A new perspective on metamorphism and metamorphic belts
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DOI：
10.1016/j.gr.2010.03.007
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发表时间：
2010-07
影响因子：
6.1
通讯作者：
S. Maruyama;H. Masago;I. Katayama;Y. Iwase;M. Toriumi;S. Omori;K. Aoki
中科院分区：
文献类型：
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作者：
S. Maruyama;H. Masago;I. Katayama;Y. Iwase;M. Toriumi;S. Omori;K. Aoki
碰撞型造山带中超高压岩石的发现，使区域变质带地表出露的进退变质作用、变质作用各阶段的年代学、变质结构成因、P-T-t轨迹、变质相系、折返模式、（7）流体在区域变质作用中的作用。主要基于我们最近对可可切塔夫、大别山、印度尼西亚、方济各会和桑巴川带的研究，我们提出了以下革命性的范式转变。在区域变质带图上定义的所谓矿物等梯度线是对俯冲过程中渐进脱水反应的误解，因为广泛的晚期水化作用已经基本上消除了泥质-砂质和变基性岩中的渐进矿物。渐进分区石榴石作为唯一的渐进矿物幸存下来，与大多数基质形成矿物不稳定。因此，经典的巴罗等梯度线应该重新仔细研究。锆石SHRIMP定年结果表明，从地幔深处到中地壳水平的缓慢折返速度为23- 40 Ma，最后阶段为造山运动和隆起。当UHP-HP单元侵入中地壳水平的低品位至未变质单元时，由于下面的流体渗透，UHP-HP单元发生了广泛的水化作用。大多数变形结构，如矿物线理，斑晶，拉分，或boudinaged角闪石，在后期广泛的水化过程中形成的不约束的渐进应力制度。利用石榴石中矿物包裹体的温压法和石榴石分带的正演模拟确定的P-T-时间轨迹，不依赖于基质矿物，在P-T空间上表现为逆时针走向，在变质相系中遵循独立的P-T变化。这与沿着Wadati-Benioff平面数值计算的地温一致。碰撞型造山带长期以来被认为具有中压型变质相系的特征。蓝晶石-硅线石是后期广泛水化作用形成的一个明显的典型相系。原始高磷-超高磷型相系为递进型，反折点在10 kb左右。碰撞型区域变质带是夹在上、下伏低磷或弱变质单元之间的一个很薄的单元。变质带的长宽比（厚度与宽度）为1：100，延伸数百至数千公里。它类似于一个薄的糜棱岩侵入体从地幔延伸到100至200公里深的地壳岩石单元。下伏单元为红柱石-硅线石型相系热变质。对碰撞型造山带中的渐进变质作用的误解主要是由于低估了来自下伏低级变质单元的流体在中地壳水平并置时的作用。流体沿着板块边界的循环比P-T变化更重要。
The discovery of ultrahigh-pressure rocks from collision-type orogenic belts has revolutionized the classic interpretation of (1) progressive and retrogressive metamorphism recorded on surface exposures of regional metamorphic belts, (2) geochronology of the various stages of metamorphism, (3) origin of metamorphic textures, (4) P–T–t path, (5) metamorphic facies series, (6) exhumation model, and (7) role of fluids during regional metamorphism. Based mainly on our recent studies of the Kokchetav, Dabie Shan, Indonesia, Franciscan and Sanbagawa belts, we suggest the following revolutionary paradigm shifts. The so-called mineral isograds defined on the maps of regional metamorphic belts were a misunderstanding of the progressive dehydration reaction during subduction because extensive late-stage hydration has mostly obliterated the progressive minerals in pelitic–psammitic and metabasic rocks. Progressively zoned garnet has survived as the sole progressive mineral that was unstable with the majority of matrix-forming minerals. The classic Barrovian isograds should therefore be carefully re-examined. The well-documented SHRIMP chronology of spot-dating zoned zircons with index minerals from low-P in the core, through HP–UHP in the mantle to low-P on the rim clearly shows that the slow exhumation speed of 23–40My from mantle depth to mid-crustal level was followed by mountain building with doming at latest stage. Extensive hydration of the UHP–HP unit occurred due to fluid infiltration underneath, when the UHP–HP unit intruded the low-grade to unmetamorphosed unit at a mid-crustal level. Most deformation textures such as mineral lineations, porphyroblasts, pull-apart, or boudinaged amphiboles, formed during extensive hydration at the late stage do not constrain the progressive stress regime. The P–T–time path determined by thermobarometry using mineral inclusions in garnet and forward modeling of garnet zoning, independent of the matrix minerals, indicates an anticlockwise trend in the P–T space, and follows an independent P–T change in the metamorphic facies series. This is consistent with the numerically calculated geotherm along the Wadati–Benioff plane. Collision-type orogenic belts have long been regarded as being characterized by the intermediate-pressure type metamorphic facies series. The kyanite–sillimanite is an apparent type facies series formed by the late-stage extensive hydration. In contrast, the original high-P to ultrahigh-P type facies series with an anticlockwise kink-point at around 10kb is a progressive type. A collision-type regional metamorphic belt crops out as a very thin unit sandwiched between overlying and underlying low-P or weakly metamorphosed units. The metamorphic belt has an aspect ratio (thickness vs width) of 1:100, and it extends for several hundreds to a thousand km. It resembles a thin mylonitic intrusion from the mantle extending from 100 to 200km depth into the crustal rock unit. The underlying unit is thermally metamorphosed in the andalusite–sillimanite type facies series. The major reason for the misunderstanding of the progressive metamorphism in collision-type orogenic belts is the underestimation of the role of fluids derived from the underlying low-grade metamorphic unit, when juxtaposed at a mid-crustal level. The circulation of fluids along the plate boundaries is more important than a P–T change.