Lithology and evolution of the crust-mantle boundary region in the southwestern Basin and Range Province
Lithology and evolution of the crust-mantle boundary region in the southwestern Basin and Range Province
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DOI：
10.1029/jb095ib01p00649
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发表时间：
1990-01
影响因子：
--
通讯作者：
H. Wilshire
中科院分区：
文献类型：
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作者：
H. Wilshire
西南部盆岭省和科罗拉多高原过渡带火山岩中的地幔和地壳捕虏体揭示了幕式岩浆作用和变形的历史，深刻地影响了这一地区的地壳结构。该地区的地震剖面显示出一般起伏度较低的强反射莫霍面，在现代剖面区，它由一个短反射层薄带（厚度<2 km）组成。上地幔是透明的，Pn为7.8-8.0 km/s，与美国西部大部分地区相似。下地壳带厚2-13公里，内反射率变化较大，速度较低，为6.6-6.8公里/秒。上地幔橄榄岩捕虏体具有韧性和脆性变形特征，其结构和成分受岩浆侵入的影响，侵入体在橄榄岩捕虏体中形成复杂的岩墙系统和广泛的晶界浸润带。而熔体渗透之前和之后的韧性变形，脆性变形，代表紧密间隔的关节系统和故障，其次是韧性变形，并与最年轻的岩浆事件。这些结构特征和上地幔温度高（> 1000°C）可能是联合收割机解释相对较低的Pn的原因。韧性变形和未变形橄榄岩的交替层，有或没有火成侵入体，可能有助于莫霍面的反射率。下地壳捕虏体主要为火成结构的辉石岩和镁铁质到中间辉长岩，与橄榄岩捕虏体中的脉岩相同。地壳捕虏体通常也是节理状的，此外，许多捕虏体显示出部分熔融，并具有丰富的空腔，这些空腔可能充满了富含CO2的流体。这些岩石被解释为底侵岩浆的产物，这些岩浆是通过地幔岩墙系统供给的，可能代表了地震记录中确定的最低地壳单元。地壳捕虏体的镁铁质成分和高密度表明，下地壳的低速可能部分是由断裂系统，部分熔融，高温。不含地幔橄榄岩捕虏体的石榴麻粒岩捕虏体可能代表晚中新世伸展前的地壳。长英质麻粒岩捕虏体从两个地方的速度一样的两个下地壳单元确定地震，并可能存在于现代地壳的不平衡残余的老地壳。下岩石圈演化的最佳模式是拉张作用影响上地幔和地壳，并在时间上被多个岩浆幕所重叠。最早期的岩浆活动先于伸展作用，晚期的岩浆活动伴随并跟随伸展作用。
Mantle and crustal xenoliths from volcanic rocks in the southwestern Basin and Range province and Colorado Plateau Transition Zone reveal histories of episodic magmatism and deformation that have profoundly influenced the crustal structure of this region. Seismic transects in this area show a strongly reflective Moho of generally low relief, which, in the area of modern transects, consists of a thin zone (<2 km thick) of short reflectors. The upper mantle is transparent and has a Pn of 7.8–8.0 km/s similar to much of the western United States. A lower crustal zone, 2–13 km thick, has variable internal reflectivity and a relatively low velocity of 6.6–6.8 km/s. Upper mantle peridotite xenoliths show both ductile and brittle deformational features and have structures and compositions affected by magmatic intrusion; intrusions form complex dike systems and extensive zones of grain boundary infiltration in peridotite xenoliths. Whereas melt infiltration preceded and followed ductile deformation, brittle deformation, represented by closely spaced joint systems and faults, followed ductile deformation and is related to the youngest magmatic episodes. These structural characteristics and high uppermost mantle temperature (∼1000°C) may combine to explain the relatively low Pn. Alternating layers of ductily deformed and undeformed peridotites, with or without igneous intrusions, may contribute to the reflectivity of the Moho. Lower crustal xenoliths are dominantly igneous-textured pyroxenites and mafic to intermediate gabbros identical to the dikes in peridotite xenoliths. The crustal xenoliths also commonly are jointed, and in addition many show partial melting and have abundant cavities that probably were filled with CO2-rich fluids. These rocks are interpreted as products of underplated magmas that were fed through the mantle dike systems and may represent the lowest crustal unit identified in the seismic records. The mafic compositions and high densities of the crustal xenoliths indicate that the low velocity of the lower crust may be caused in part by fracture systems, partial melts, and high temperatures. Garnet granulite xenoliths from a locality with no mantle peridotite xenoliths probably represent crust of the region before late Miocene extension. Felsic granulite xenoliths from two localities have velocities like those of the two lower crustal units identified seismically and could be present in the modern crust as unequilibrated remnants of old crust. The preferred model for the evolution of the lower lithosphere is one in which extension affects the upper mantle as well as the crust and is overlapped in time by multiple magmatic episodes. The earliest magmatic events preceded extension, and later events accompanied and followed extension.