Basalt

About: Basalt is a research topic. Over the lifetime, 18687 publications have been published within this topic receiving 805136 citations.

A conceptual hypothesis for the relation of differing tectonic terranes to plutonic emplacement

Abstract: Great batholiths such as those formed in western North America during the past 200 m.y. resulted from major additions of sialic rock to the crust and consequent volumetric and areal expansion of the sialic crust. Such crustal growth begins with the emplacement of rock of basaltic composition at the base of the crust. From this underplating, differentiated magmas rise as diapirs into the upper crust. Some of this material escapes upward into the zone of brittle deformation and forms shallow plutons and volcanic ejecta, but the greater portion comes to rest at depths of 5 to 15 km as essentially conformable plutons. The volumetric additions caused by the emplacement of great volumes of rock within these depth limits are made possible by lateral compression and transport of the pre-existing upper crustal rocks. To accommodate lateral compressive transport at intermediate depths, supra-crustal rocks undergo lateral extension by block faulting, and deep crustal rocks extend by lateral flow. Zones of extension, lateral compression, and so-called load flattening—where they exist within belts of batholith emplacement—have probably formed simultaneously one above the other.

Clast assemblages of possible deep-seated /77517/ and immiscible-melt /77538/ origins in Apollo 17 breccias

Abstract: Breccia samples 77517 and 77538 are composed of abundant mineral and lithic clasts set in porous, poorly sintered matrices Clast assemblages in the two rocks are of contrasting composition and origin Breccia 77517 has Mg-rich olivine and pyroxene and calcic plagioclase clasts, indicating limited, almost exclusively ANT-suite parentage A significant feature is the presence of an assemblage (aluminous enstatite, forsterite, anorthite, aluminous spinel) corresponding to spinel cataclasite, a rock type of deep-seated (about 60 km) crustal origin Breccia 77538 contains Fe-rich pyroxene and rather sodic plagioclase clasts, indicative of predominantly KREEP and/or mare derivation An important feature is the occurrence of high-K and high-Fe lithic clasts whose compositions resemble those of immiscible-melts produced during late-stage magmatic crystallization, and which probably originated via silicate liquid immiscibility in a KREEP or mare basalt magma Both rocks contain numerous fine-grained breccia clasts which represent material that has been modified by impact processes at or very near the moon's surface

Volcanic Pipes as Clues to Upper Mantle Petrogenesis: Mesozoic Ar-Ar Dating of the Minusinsk Basalts, South Siberia

Abstract: 40Ar/39Ar dating of alkali basalt pipes and dikes of the North Minusinsk basin has provided an important chronologic framework for Altay-Sayan fold belt and Central Asia region. These Mesozoic basalt pipes have become the subject of special interest due to the abundant occurrence of garnet-spinel and spinel lherzolite xenoliths. These rocks reveal information about the composition, structure, and thermal state of the Mesozoic upper mantle beneath the southwestern margin of the Siberian craton. Our detailed 40Ar/39Ar dating has shown that the pipes and their related NW-trending dikes formed coevally, during a Late Cretaceous, short-time, magmatic impulse. Twelve plateau ages on feldspar megacryst and whole rocks between 72 ±2.7 and 79 ±2 Ma have been determined. Abundant E-W-trending dikes, previously though to be related to the Meso-Cenozoic subvolcanic complex, have a less alkaline composition, compared to the basanite pipes, with Early Permian 40Ar/39Ar ages. The trachyte NW-trending dikes yielded an Ea...

Lithosphere-asthenosphere mixing in a transform-dominated late Paleozoic backarc basin Implications for northern Cordilleran crustal growth and assembly

Abstract: The Slide Mountain terrane is part of a North American Cordillera–long backarc basinal assemblage that developed between the ensialic arc terranes (Yukon-Tanana and affiliated pericratonic terranes) and the North American craton in the middle to late Paleozoic. The Slide Mountain basin started to open in the Late Devonian, and spreading continued through the late Paleozoic in an oblique (transform-dominated) manner such that the pericratonic terranes were translated into southerly latitudes. The basin closed, also in an oblique manner, by the Early Triassic, resulting in the reaccretion of the Yukon-Tanana terrane to the northwestern Laurentian margin. Both the opening and closing likely involved hundreds to possibly thousands of kilometers of intra-ocean and/or intra-arc strike-slip displacement, sinistral during the ocean9s Late Devonian to mid-Permian opening and dextral during its Late Permian closing. In southeastern Yukon, Canada, the Early Permian Slide Mountain terrane is dominated by mafic and ultramafic volcanic and plutonic rocks of the Campbell Range Formation. These rocks are narrowly distributed, for over 300 km, on either side of the Jules Creek–Vangorda fault, a fault that separates Slide Mountain terrane from Yukon-Tanana terrane. The Campbell Range basaltic volcanic and high-level intrusive rocks have geochemical and isotopic signatures that vary systematically across the Jules Creek–Vangorda fault: ocean-island basalt (OIB) and enriched mid-ocean ridge basalt (E-MORB) suites with lower eNd t occur exclusively south of the fault, whereas north of the fault they have normal mid-ocean ridge basalt (N-MORB) and backarc basin basalt (BABB) signatures with higher eNd t values. The eNd t values are inversely correlated with Nb/Th pm and Nb/La pm , suggesting that the lower eNd t values present in the E-MORB and OIB are mantle source features of these basalts and not due to continental crustal contamination. Isotopic and multi-element mixing calculations illustrate that the OIB-like basalts were derived primarily from enriched continental lithospheric mantle, whereas the N-MORB and BABB suites were sourced primarily from the upwelling backarc asthenospheric mantle; E-MORBs represent mixtures of depleted asthenospheric and enriched lithospheric mantle. The geochemical and isotopic variations in the Campbell Range Formation across the Jules Creek–Vangorda fault is attributed to formation in different parts of an extending continental-backarc basin and then their subsequent juxtaposition by continued displacement along the fault. Despite the juvenile isotopic signatures present in the Slide Mountain terrane, they occur as thin klippe atop rocks of recycled continental crustal affinity, suggesting that they were likely only minor contributors to Cordilleran crustal growth.