Partial melting of amphibolite/eclogite and the origin of Archean trondhjemites and tonalites
Partial melting of amphibolite/eclogite and the origin of Archean trondhjemites and tonalites
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DOI：
10.1016/0301-9268(91)90092-o
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发表时间：
1991-06
影响因子：
3.8
通讯作者：
Robert P. Rapp;E.Bruce Watson;Calvin F. Miller
中科院分区：
文献类型：
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作者：
Robert P. Rapp;E.Bruce Watson;Calvin F. Miller
太古宙（3.8 ~ 2.5 Ga）大规模的闪长岩和闪长岩的生成标志着岩浆地壳向硅质地壳的转变，代表了岩浆对克拉通化的贡献。这些岩石的成因模型基于它们的高分馏、贫稀土模式，表明它们是基性地壳来源，要么是角闪石、石榴石-角闪石或榴辉石的部分熔融过程，其中角闪石和/或石榴石是主要的残余相，要么是角闪石控制的含水玄武岩岩浆分馏。在8、16、22和32 kbar条件下，对4种天然玄武岩成分进行了一系列无蒸汽（即P流体< P总）熔融实验，以评估太古宙花岗岩成因模型的有效性，这些模型假设太古宙花岗岩为基性地壳源。在所有压力下，10-40%熔融产生的熔体成分均为调性-长闪质；剩余组合为角闪孔+ plag±opx±FeTi (8 kbar)，石榴石+ cpx±角闪孔±plag±opx±FeTi氧化物（16 kbar），石榴石+ cpx±金红石（22和32 kbar）。根据估算的熔体和残余相的模态比例计算，当残余中存在一定程度的石榴石时，大多数长闪-调性部分熔体的REE模式是高度分馏的（layb为30-50），重稀土（ybn为1-10）；这些稀土元素模式与太古宙“灰色片麻岩”和花岗绿岩地区的长闪质和调性片麻岩相似。根据实验P- t条件考虑估计的太古宙地热，表明一个暂时减少的太古宙地热可能逐渐穿过一个P- t区，在这个区中，通过在5-8 kbar的部分熔融，从水饱和（P f= P t）到不饱和（P f< P t）角闪岩源最初可能产生长闪质-调性熔体。在太古宙中晚期，随后的地热松弛将产生类似的熔体，即16kbar的石榴石角闪岩和22-32 kbar的榴辉岩的无蒸汽熔化。然而，产生闪石-调性组成的熔体所需的熔化程度随着压力的增加而增加，以角闪石为主的残渣在8 kbar时熔化10-15%合适，但在22-32 kbar时熔化25-35%，其中石榴石为主。假设熔体分离的趋势和/或岩浆动员机制在较高的熔融程度下更有效，则榴辉岩部分熔融的成因似乎是大量深成闪长-闪长-闪长岩对幼年大陆贡献的最有可能的来源。这种来源与在任何可能的太古宙构造-热情景中形成的长闪岩-闪岩原大陆岩心是一致的，尽管不一定是在常规的俯冲带环境中。
The generation of trondhjemites and tonalites on a massive scale during the Archean (3.8-2.5 Ga ago) marked the transition from a simatic to a sialic crust, and represents the magmatic contribution to cratonization. Petrogenetic models for the origin of these rocks based on their highly fractionated, HREE-depleted rare earth patterns suggest a mafic crustal source, either through a process of partial melting of amphibolite, garnet-amphibolite, or eclogite, in which hornblende and/or garnet are essential residual phases, or by hornblende-controlled fractionation of hydrous basaltic magma. A series of vapor-absent (ie, P fluid< P total) melting experiments on four natural basaltic compositions were conducted at 8, 16, 22 and 32 kbar in order to assess the validity of models for the origin of Archean granitoids which assume a mafic crustal source. Melt compositions produced by 10–40% melting are tonalitic-trondhjemitic at all pressures investigated; residual assemblages are amphibole+ plag±opx±Fe Ti at 8 kbar, garnet+ cpx±amphibole±plag±opx±Fe Ti oxide at 16 kbar, and garnet+ cpx±rutile at 22 and 32 kbar. REE patterns for most of the trondhjemitic-tonalitic partial melts, calculated on the basis of estimated modal proportions of melt and residual phases, are highly fractionated (La Yb is 30–50), heavy rare earth-depleted (Yb N is 1–10) when garnet is present to some extent in the residue; these REE patterns are similar to those of trondhjemitic and tonalitic gneisses from several Archean “grey gneiss” and granite-greenstone terrains. A consideration of estimated Archean geotherms with respect to the experimental P-T conditions indicates that a temporally diminishing Archean geotherm might have progressively swept through a P-T regime in which trondhjemitic-tonalitic melts could have been generated initially from a water-saturated (P f= P t) to undersaturated (P f< P t) amphibolite source by partial melting at 5–8 kbar. Subsequent relaxation of the geotherm through the mid-to late-Archean would have produced similar melts by vapor-absent melting of garnet-amphibolite at 16 kbar and eclogite at 22–32 kbar. However, the degree of melting required to produce melts of trondhjemitic-tonalitic composition increases with pressure, 10–15% melting being appropriate at 8 kbar in a amphibole-dominated residue, but 25–35% melting being required at 22–32 kbar, where garnet dominates the residue. Supposing the tendency for melt segregation and/or magma mobilization mechanisms to be more effective at higher degrees of melting, an origin by partial melting of eclogite seems to be the most likely source for massive plutonic trondhjemite-tonalite contributions to the juvenile continents. Such a source is consistent with the generation of trondhjemite-tonalite protocontinental cores in any number of plausible Archean tectono-thermal scenarios, though not necessarily in a conventional subduction-zone setting.