Ultramafic pseudotachylite from the Balmuccia peridotite, Ivrea-Verbano zone, northern Italy
Ultramafic pseudotachylite from the Balmuccia peridotite, Ivrea-Verbano zone, northern Italy
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DOI：
10.1016/0040-1951(94)00228-2
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发表时间：
1995-02
期刊：
影响因子：
2.9
通讯作者：
M. Obata;S. Karato
中科院分区：
文献类型：
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作者：
M. Obata;S. Karato
本文报道了在意大利北方Ivre-Verbano带的尖晶石二辉橄榄岩体中发现的超镁铁质假玄武玻璃岩。假玄武玻璃岩以不协调的脉岩和脉状横切橄榄岩的原生结构和剪切带中部的薄层（约1 cm）形式出现。假玄武玻璃岩具有类似于寄主橄榄岩的超镁铁质成分。在薄片上，它由细粒自形橄榄石和单斜辉石晶体组成，这些晶体密集地堆积在玻璃质基质中，并含有少量的碎屑橄榄石颗粒和岩屑糜棱岩碎屑。利用TEM对橄榄石进行显微结构观察，得到围岩和橄榄石碎屑在10300 MPa的差应力水平下变形的证据。岩石学分析的主机岩石和pseudotachylite结合剪切加热的能量计算表明，pseudotachylite起源于近完全熔融的主机橄榄岩，然后快速结晶的时间尺度为100秒或更少，条件符合地震。超镁铁质假玄武玻璃岩的存在意味着一个非常显着的剪切加热，可能是由于断层面上的高剪切应力。与熔融有关的超镁铁岩中的断层将导致大的应力降和由此产生的大的力矩释放，因为由于超镁铁熔体的粘度非常低，在熔融时滑动阻力几乎完全丧失。
This paper reports an occurrence of ultramafic pseudotachylite from a spinel Iherzolite mass in the Ivre-Verbano zone, northern Italy. The pseudotachylite occurs as discordant dikes and veins crosscutting primary structures of the peridotite and as thin (∼ 1 cm) layers in the middle of shear zones. The pseudotachylite has an ultramafic composition similar to the host peridotite. In thin section it consists of fine-grained euhedral olivine and clinopyroxene crystals densely packed in a glassy matrix with minor amounts of clastic olivine grains and lithic mylonitic clasts. Microstructural observations on the olivines using TEM yields evidence for deformation in the wall rock and olivine clasts at a differential stress level of ∼ 300 MPa. Petrological analysis of the host rock and pseudotachylite combined with an energy calculation for shear heating suggests that the pseudotachylite originated by a near total melting of the host peridotite followed by rapid crystallization on a time scale of 100 s or less, conditions consistent with an earthquake. The presence of an ultramafic pseudotachylite implies a very significant shear heating due presumably to a high shear stress on the fault plane. Faulting in an ultramafic rock associated with melting will result in a large stress drop and resultant large moment release, because resistance to sliding will almost completely be lost upon melting due to the very low viscosity of ultramafic melts.