Early Proterozoic bimodal volcanic rocks in central Colorado,U.S.A., Part II: Geochemistry,petrogenesis and tectonic setting

The early Proterozoic supracrustal rocks of the Salida area of central Colorado consist of strongly bimodal sequences of volcanogenic rocks. The mafic rocks — basalts, basaltic volcaniclastics, and related gabbro sheets — are distinctly tholeiitic, display a strong iron-enrichment trend, and typically contain less than 50% SiO₂. The felsites are rhyolites to dacites and contain more than 70% SiO₂.Major and trace element modeling show that the mafic rocks underwent two stages of crystal fractionation, the first involving olivine and plagioclase, the second involving plagioclase and clinopyroxene. Fractionation occurred within individual injections as they rose toward the surface rather than in a single magma chamber at depth. Field relations and major element data support the derivation of the felsic rocks from a magma generated by anatexis of sialic crust. However, the low Sr and high heavy REE concentrations in these rocks are not compatible with a partial melting model and suggest that the felsic volcanic rocks could have been derived by extensive fractional crystallization of the mafic magma.Normalized trace element abundances and trace element ratios of the mafic rocks are most like continental flood basalts such as the Columbia River basalts. They also display some similarity to immature back-arc basin tholeiites developed on continental crust, such as those of the Sarmiento complex. The felsic rocks have strong chemical affinities to within-plate rhyolites rather than calc-alkaline rhyolites from orogenic areas. The chemical data, as well as the petrographic, stratigraphic, and regional field data all indicate that the early Proterozoic supracrustal rocks of the Salida area developed along a continental margin, probably within an immature back-arc basin underlain by sialic crust. Remnants of the arc system of similar age may lie to the south in northern New Mexico and southwestern Colorado.     

Early Proterozoic bimodal volcanic rocks in central Colorado,U.S.A., Part I: Petrography,stratigraphy and depositional history

Early Proterozoic metamorphic rocks of the Salida area include two exceptionally well-preserved sections of undeformed volcanogenic rocks: a northern section nearly 4000 m thick and a southern section 1700 m thick. The stratigraphic and lithologic character of the sections are the same, indicating that they were formed contemporaneously in the same subaqueous environment. The suite is strongly bimodal, consisting of felsic volcaniclastic rocks (50%), mafic volcanic and volcaniclastic rocks (22%), and gabbro/diabase sheets (23%). The remaining rocks are mostly pelitic metasediments.The felsic rocks, which dominate the upper portions of both sections, comprise four distinct facies: crystal-lithic tuff breccia, crystal-lithic lapilli-breccia tuff, bedded tuff, and massive tuff. Lithic fragments are typically foreign, dominated by pelitic material. Crystal clasts are feldspar, mainly plagioclase, and quartz. Sequences of these facies, representing cycles of eruptive activity, are uniform in thickness and extend along strike for the entire width of Precambrian exposure, up to 4 km, but individual facies units, especially the coarser ones, are limited in their lateral extent. Individual sequences average about 100 m thick, but continuous exposures of felsic rocks locally exceed 600 m. Primary sedimentary structures and vertical and lateral variations indicate deposition, typically following remobilization as debris flows and turbidites, in a shallow marine environment of low relief.Mafic volcanic and volcaniclastic rocks form accumulations up to 600 m thick and extend uniformly for several kilometers along strike, although individual units are more restricted laterally. The principal rock types are basalt flows, basaltic lithic breccia, pillow lava/pillow breccia, bedded basaltic lapillistone, and laminated basaltic tuff, all deposited in a submarine setting. Accumulations, typically including several types, are interbedded with felsic rocks, but mechanical mixing of mafic and felsic volcaniclastics is rare, suggesting separate eruptive centers for the mafic and felsic volcanics. The northern section may be more proximal than the southern section, as indicated by the greater abundance of flows, the thicker sheets of gabbro, and the greater abundance of mafic rocks relative to felsic rocks.Gabbro/diabase sheets, up to 350 m thick and dominantly concordant, intrude throughout both sections. They are characterized by lack of internal differentiation, simple mineralogy consisting of plagioclase and hornblende, pseudomorphous after pyroxene, and sub-ophitic texture. The minor pelitic material indicates that the area was accessible to continental sediment sources, although their location is not known.The Salida bimodal volcanic suite is part of a more extensive accumulation of similar 1700–1800 Ma supracrustal rocks that extends west to the Gunnison area and perhaps south into northern New Mexico.     

The origin of the albite-rich rocks enclosing the cobaltian pyrite deposit at Thackaringa,N.S.W., Australia

A stratabound disseminated and massive cobaltian pyrite deposit at Thackaringa, 30 km SW. of Broken Hill occurs in banded albite-quartz-biotite rocks which are conformable with regional structure and stratigraphy. The albite rocks are associated with pelitic and psammitic metasediments, amphibolite and minor quartzgahnite rocks. The deposit has undergone granulite facies metamorphism and there is no evidence that pyrite has reacted to form pyrrhotite. It is suggested that the albite rocks were an analcimized tuffaceous rock in which disseminated pyrite formed by a volcanic exhalative process. A massive cobaltian pyrite body is enclosed by a metamorphosed hydrothermal alteration zone characterised by an increase in quartz, magnesian fluorobiotite and Rb.     

西南天山特克斯科桑溶洞火山岩的锆石U-Pb年代学研究

新疆特克斯县乌孙山科桑溶洞火山-沉积岩剖面由变质岩、火山-沉积岩(中酸性火山熔岩、火山碎屑岩、生物碎屑灰岩)和侵入其中的花岗质岩石组成,花岗岩中保留着英安岩-安山岩顶垂体。夹在火山岩地层之间的厚层生物碎屑灰岩(其中发育溶洞)发生褶皱变形。在剖面多处见到逆冲构造,断裂带局部塑性变形。采自剖面下部英安岩样品的锆石232Th/238U比值为0.4～0.6,U-Pb谐和年龄368.3±1.7Ma(MSWD=1.5,n=15)。剖面上部流纹岩中锆石的232Th/238U比值为0.8～2.2,U-Pb谐和年龄为356.4±1.7Ma(MSWD=1.3,n=19)。大约12Ma的时差说明存在两次火山喷发事件,在这两次火山喷发的间歇形成了巨厚生物碎屑灰岩(1000m)。这种间歇性的火山喷发在西天山地区普遍发育,形成了规模巨大的滨海相火山-沉积盆地。剖面下部的英安岩与剖面上部的流纹岩的锆石具有不同的微量元素组成,预示着其岩浆源区性质的差异。     

流体晶、流体晶矿物组合、流体岩及其研究意义

流体是地球各圈层之间相互作用的纽带,在成岩、成矿过程中起着十分重要的作用。目前,流体的研究主要集中在流体对先存矿物岩石进行的交代作用方面,而对流体直接结晶形成的矿物领域研究较少。文中根据作者近几年的研究成果对从流体直接结晶而成的矿物——流体晶以及流体晶矿物组合、流体岩等的定义、特征进行了归纳总结。最新的研究结果显示：岩浆中可以含有大量的流体,这些流体来源既可以是岩浆演化富集、岩浆与围岩相互作用产生,亦可以是外部来源。因此,流体晶矿物、流体岩在自然界应该是普遍存在的。流体晶矿物的提出将深化人们对地质过程的理解,发展岩石学及矿床学的研究新领域,有利于矿床勘探和成矿预测。