Relationships between crustal partial melting, plutonism, orogeny, and exhumation: Idaho–Bitterroot batholith
Relationships between crustal partial melting, plutonism, orogeny, and exhumation: Idaho–Bitterroot batholith
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DOI：
10.1016/s0040-1951(01)00169-x
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发表时间：
2001-12
期刊：
影响因子：
2.9
通讯作者：
D. Foster;C. Schafer;C. Fanning;D. W. Hyndman
中科院分区：
文献类型：
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作者：
D. Foster;C. Schafer;C. Fanning;D. W. Hyndman
爱达荷州北部-比特鲁特岩基地区是一个挖出的中地壳深成-变质杂岩体，是在地壳增厚和随后在科迪勒拉造山带腹地的伸展过程中形成的。该地区变质作用、部分熔融、侵入和变形的相对时间可能为现代造山带中地壳深处活跃的岩浆和变形过程提供类比。科迪勒拉山脉这一部分的地壳增厚在 100 Ma 之前就开始了，但这一过程的早期阶段受到的限制很少。 U-Pb 锆石测年结果表明，高级变质作用与约 1000 年同构造石英闪长岩岩体的侵入同时发生。 75–80 Ma，位于比特鲁特变质核复合体中，地壳最深处暴露于此。核杂岩东北部的变质条件达到～0.65–0.75 GPa，温度～600–750 °C。上部角闪岩相条件导致石英长石片麻岩广泛部分熔融，而下面的半泥质片岩中的白云母分解促进了这种熔融。新的和先前发表的 U-Pb 锆石年龄表明，下地壳和中地壳的主要部分熔融发生在大约 10 到 10 年间。 65 和 53 Ma，导致大量“主相”花岗岩岩体侵入厚（3-4 公里）的岩床。侵入伴随着新的上角闪岩相变质作用和在~0.65 GPa压力下部分熔融形成混合岩。最年轻的中地壳花岗岩侵入体的年龄与造山带最初塌陷和伸展的年龄大致相同。 52–50 Ma。延伸主要发生在比特鲁特糜棱岩带上，该带使较新的侵入体以及较古老的高品位岩石变形。因此，中下地壳的大规模部分熔融伴随着地壳增厚达15-35 Ma，但仅提前了1-3 Ma的伸展和折返。科迪勒拉造山带这一区域的崩塌似乎集中在部分熔融和深成作用最为强烈和持续时间最长的地方。从角闪岩相糜棱岩化、到绿片岩相剪切、到脆性断层、到剪切带不活动的转变揭示了折返作用，剪切带从西部较浅的地壳水平进展到东部较深的地壳水平。 51–38 Ma，基于 U-Pb 和 Ar-Ar 结果。碱长石花岗岩是在剥露开始时就位的，但仅侵入最浅的始新世地壳层。它们的产生可能与地壳减薄期间岩石圈柱的减压有关。
The northern Idaho–Bitterroot batholith region is an exhumed, mid-crustal, plutonic–metamorphic complex that formed during crustal thickening and subsequent extension in the hinterland of the Cordilleran orogen. The relative timing of metamorphism, partial melting, intrusion, and deformation in this area may provide an analogue for magmatic and deformation processes active at mid-crustal depths in modern orogenic belts. Crustal thickening in this part of the Cordilleran began before 100 Ma, but the early stages of this process are poorly constrained. U–Pb zircon dates indicate that high-grade metamorphism was coincident with intrusion of syntectonic quartz diorite plutons at ca. 75–80 Ma, in the Bitterroot metamorphic core complex, where the deepest crustal levels are exposed. Metamorphic conditions reached ∼0.65–0.75 GPa and ∼600–750 °C in the northeastern part of the core complex. Upper amphibolite facies conditions caused widespread partial melting in quartzofeldspathic gneiss facilitated by muscovite breakdown in underlying semi-pelitic schist. New and previously published U–Pb zircon ages indicate that major partial melting of the lower and middle crust occurred between ca. 65 and 53 Ma, leading to the intrusion of the voluminous “main-phase” granitic plutons as thick (3–4 km) sills. Intrusion was accompanied by renewed upper amphibolite facies metamorphism and partial melting forming migmatites at ∼0.65 GPa pressure. The youngest mid-crustal granitic intrusions are about the same age as initial collapse of the orogen and extension at ca. 52–50 Ma. Extension was accommodated mainly on the Bitterroot mylonite zone that deforms the younger intrusions as well as the older high-grade rocks. Therefore, large-scale partial melting of the middle and lower crust followed crustal thickening by as much as 15–35 Ma, but pre-dated extension and exhumation by only 1–3 Ma. Collapse in this sector of the Cordilleran orogen appears to have been focussed where partial melting and plutonism were most intense and long-lived. Exhumation is revealed by the transition from amphibolite facies mylonitization, to greenschist-facies shearing, to brittle faulting, to inactivity of the shear zone that progressed from shallower crustal levels in the west to deeper crustal levels in the east from ca. 51–38 Ma, based on U–Pb and Ar–Ar results. Alkali-feldspar granites were emplaced during the onset of exhumation, but were intruded only into the shallowest Eocene crustal levels. Their generation may have been linked to decompression of the lithospheric column during crustal thinning.