Petrography

About: Petrography is a research topic. Over the lifetime, 7449 publications have been published within this topic receiving 102018 citations.

Thermobarometry in a subgreenschist to greenschist transition in metabasites of the Abitibi greenstone belt, Superior Province, Canada

Abstract: Mineral equilibria in the system CaO–MgO–Al2O3–SiO2–H2O provide a basis for mapping of four reaction isograds and one bathograd in the low-pressure transition from subgreenschist to greenschist facies. Most of the Matachewan area of the Abitibi greenstone belt is in the lower-pressure bathozone, as indicated by the widespread occurrences of the subassemblage Prh–Chl. The higher-pressure bathozone is indicated by two occurrences of Pmp–Act–Ep–Qtz, but in these samples the bathograd is displaced to anomalously low pressure by the high Fe content of the coexisting minerals. This illustrates the need to analyse coexisting minerals, calculate activities of end-member species, and compute P–T curves for individual samples before interpreting the isograd/bathograd pattern. Petrographic and microprobe analysis indicates that great care must be taken in the selection of ‘equilibrium’ assemblages. Pyroxene phenocrysts in one sample are replaced by the assemblage Pmp–Act–Ep–Chl–Qtz, whereas Prh–Act–Ep–Chl–Qtz occurs in the groundmass. Compositional variation may be more cryptic, as in a sample of metabasaltic hyaloclastite that contains two spatially distinct ‘univariant’ assemblages, Prh–Pmp–Ep–Chl–Qtz and Prh–Act–Ep–Chl–Qtz, within the devitrified matrix. Whereas chlorite compositions are similar in both assemblages, prehnite and epidote in the latter assemblage are significantly richer in Fe and poorer in Al. Accordingly, the rock is interpreted to contain two distinct ‘univariant’ assemblages, rather than one ‘invariant’ assemblage (Prh–Pmp–Act–Ep–Chl–Qtz). The displaced ‘univariant’ curves for this sample intersect at 2.2 kbar and 250°C. Taking account of all thermobarometric implications, the low-grade limit of the greenschist facies is at 250–270°C and 2–2.5 kbar, corresponding to depths of 7–8 km. Comparison of apparent P–T conditions on both sides of the Larder Lake – Cadillac break, a regional CO2-metasomatized fault zone that is spatially associated with many Archaean gold deposits, provides an upper limit of not more than c. 1 km for post-metamorphic south-side-up, dip-slip displacement.

RbSr geochronology and geochemistry of torlesse metasediments from the Central North Island, New Zealand

Abstract: Petrographic, chemical and isotopic data are presented for a suite of metasedimentary rocks from the Torlesse terrane, North Island, New Zealand. Detrital mineral modes and chemical trends indicate that the rocks are the end-products of hydraulic sorting of dominantly granitic detritus by turbidity currents. Subsequent diagenetic and metamorphic alteration has only slightly modified the original bulk-rock chemistry although important mineralogical changes have occurred. A RbSr whole-rock isochron for the argillitic end-members of the suite gives an age of 141 ± 3 Ma and is interpreted to represent the timing of low-grade metamorphism of the rocks. The study presents evidence to show that near-complete equilibration of Sr isotopes is possible in coarse-grained greywackes as well as in fine-grained argillites during prehnite-pumpellyite to lower-greenschist facies metamorphism. The process relies mainly on the albitisation of plagioclase and recrystallisation of the matrix accompanied by interaction of the released Sr with circulating pore fluids within relatively small domains.

Petrography, geochemistry, and tectonics of a rifted fragment of Mainland Asia: evidence from the Lasala Formation, Mindoro Island, Philippines

Abstract: Petrological and geochemical investigations of the sedimentary Lasala formation in northwest Mindoro, Philippines, offer new insights into the origin of this geologically contentious region. Mindoro island’s position at the boundary between Sundaland and the Philippine Mobile Belt has led to variable suggestions as to how much of it is continent derived or not. The Eocene Lasala formation overlies the Jurassic Halcon metamorphics, a regionally metamorphosed suite generally thought to have formed as a result of arc-continent collision processes. The sedimentary formation consists mainly of sandstones and shales interbedded with mudstones, basalt flows, and subordinate limestones and conglomerates. Petrographic information on the Lasala clastic rocks demonstrates a uniform framework composition that is predominantly quartzose. Major oxide, trace element abundances, and various elemental ratios similarly impart a strongly felsic signature. These characteristics are taken to indicate a chiefly continental, passive margin derivation and deposition of the Lasala sediments during the Eocene. The weak indication of active margin influence is suggested to be an inherited signature, supported by paleogeographic models of the southeastern Asian margin area during the pre-Cenozoic.

Cementing Materials in Sandstones and Their Probable Influence on Migration and Accumulation of Oil and Gas

Abstract: Detailed petrographic studies of sandstone cores from productive and non-productive structures in the Rocky Mountain region were made to determine not only the physical characteristics of the sandstones but also the nature and distribution of the cementing materials. The sandstones have been divided into two general groups on the basis of the cementing or binding materials: first, those in which the grains are bound together by argillaceous materials; and second, those in which crystalline minerals form the cement or bond. In the first group, effects of mechanical compaction were noted. In the second group, sequence of deposition of cementing minerals was observed. The deposition of quartz, dolomite, and anhydrite, in the order given, is common in some sandstones. In othe s, quartz and calcite are the only cementing minerals, and of these, calcite is the last mineral deposited. Other combinations of these four minerals were observed. Furthermore, sequence of deposition of the same minerals has been established tentatively for the geologic section from the Bell sandstone which is the basal member of the Minnelusa formation (Pennsylvanian) upward through the Mesaverde (Upper Cretaceous) sandstone. Also presented are some probable effects of crystalline cementing minerals upon pressures existing in oil and gas fields, the probable control exercised by these mineral precipitates upon the movement of fluids in reservoir rocks between areas of cementation and pressure relief, and the relationship of this fluid movement to the migration and accumulation of oil and gas.