Craton nucleation and formation of thick lithospheric roots
Craton nucleation and formation of thick lithospheric roots
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DOI：
10.1016/j.lithos.2012.02.011
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发表时间：
2012-09
期刊：
影响因子：
3.5
通讯作者：
S. Aulbach
中科院分区：
文献类型：
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作者：
S. Aulbach
克拉通作用被认为与碰撞构造过程中的融合作用和岩石圈增厚相吻合，导致了我们今天所知的克拉通实体的诞生。然而，它们本身是早期板块构型的裂解残留物，涉及克拉通核，其形成模式制约着它们的性质，因此稳定性和进一步的演化。对钻石包裹体的测年结果表明，一些厚重的克拉通根早于导致克拉通混杂的增生过程而存在，或反映在克拉通地幔中榴辉岩的年龄中。主要的不确定性涉及：(1)这些核是在环境或超高地幔位温Tp下形成的，即在大洋中脊或大洋高原环境中形成的；(2)深含石榴石的克拉通地幔，其中一部分延伸到钻石稳定区(>；150公里)，是由大洋岩石圈的羽流俯冲或堆积形成的。为了从公布的数据集中提取约束条件，其中包括经地幔交代作用修改的样品，钻石中的橄榄石包裹体被用作克拉通次大陆岩石圈地幔(SCLM)的交代前成分的指南。过滤数据集的FeO-MgO关系表明，许多枯竭的克拉通SCLM剖面是在5 Gpa压力下部分熔融的开始而成核的，可能指向典型的羽流形成，而不是太古代中脊环境的形成。Cr_2O_3-Al_2O_3关系制约着平衡部分熔体萃取压力。所有克拉通亏损石榴石橄榄岩套的成分都与压力≥3 Gpa时的地层一致，因此是就地形成的，而不是洋板俯冲形成的。因为大多数克拉通SCLM切片是由橄榄石和铝斜方辉石组成的高压残留物形成的，所以在压力为3 GPa时最后平衡的残馀橄榄岩中没有石榴石特征，并不能证明尖晶石稳定区的原始部分熔融。克拉通地壳可能是由浅层板块相互作用形成的，导致了TTGs和绿岩带的产生。这些地壳核可能随后被来自羽流的熔体穿透和埋入地下，并在致密的榴辉岩和科马提岩剥离后，被浮力羽流残留物捕获。这确保了克拉通的寿命，并暗示克拉通的地壳和地幔并不完全是同源的。
Cratonisation is regarded as coincident with amalgamation and lithosphere thickening during collisional tectonics, leading to the birth of the cratonic entities we know today. These are, however, themselves rifted remnants of earlier plate configurations involving cratonic nuclei whose mode of formation conditioned their properties, hence stability and further evolution. Dating of inclusions in diamonds reveals that some thick cratonic roots existed well in advance of accretionary processes leading to craton amalgamation or reflected in the ages of eclogites residing in cratonic mantle. Major uncertainties relate to (1) whether these nuclei formed at ambient or excess mantle potential temperatures TP, i.e. in mid-ocean ridge or oceanic plateau settings and (2) whether the deep, garnet-bearing cratonic mantle, a portion of which extends into the diamond stability field (>150km), formed by plume subcretion or stacking of oceanic lithosphere. In order to extract constraints from published data sets that include samples modified by mantle metasomatism, olivine inclusions in diamonds are used as a guide to the pre-metasomatic composition of the cratonic subcontinental lithospheric mantle (SCLM). The FeO–MgO relationships of the filtered data set show that many depleted cratonic SCLM sections nucleated by onset of partial melting at pressures>5GPa, possibly pointing to formation at excess TPtypical of plumes, rather than at ambient TPin Archaean mid-ocean ridge settings. Cr2O3–Al2O3relationships constrain pressures of equilibrium partial melt extraction. All cratonic depleted garnet peridotite suites have compositions consistent with formation at pressures≥3GPa and hence formed in situ and not by subduction of oceanic plates. Because most cratonic SCLM sections formed as high-pressure residues consisting of olivine and aluminous orthopyroxene, the absence of a garnet signature in residual peridotites that last equilibrated at pressures >3GPa is not proof for original partial melting in the spinel stability field. Cratonic crust may have formed by shallow plate interactions leading to generation of TTGs and greenstone belts. These crustal nuclei may have been subsequently penetrated and subcreted by plume-derived melts, and, after delamination of dense eclogite and komatiites, were trapped by buoyant plume residues. This ensured craton longevity and implies that cratonic crust and mantle are not entirely cogenetic.