Petrography

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

Petrographic evidence shows that pottery exchange between the Olmec and their neighbors was two-way

Abstract: Petrographic thin sections of pottery from five Formative Mexican archaeological sites show that exchanges of vessels between highland and lowland chiefly centers were reciprocal, or two-way. These analyses contradict recent claims that the Gulf Coast was the sole source of pottery carved with iconographic motifs. Those claims were based on neutron activation, which, by relying on chemical elements rather than actual minerals, has important limitations in its ability to identify nonlocal pottery from within large data sets. Petrography shows that the ceramics in question (and hence their carved motifs) have multiple origins and were widely traded.

The Alkaline–Peralkaline Tamazeght Complex, High Atlas Mountains, Morocco: Mineral Chemistry and Petrological Constraints for Derivation from a Compositionally Heterogeneous Mantle Source

Abstract: The Eocene Tamazeght complex, High Atlas Mountains, Morocco is a multiphase alkaline to peralkaline intrusive complex. A large variety of rock types (including pyroxenites, glimmerites, gabbroic to monzonitic rocks, feldspathoidal syenites, carbonatites and various dyke rocks) documents a progression from ultramafic to felsic magmatism. This study focuses on the silicate plutonic members and the genetic relationships between the various lithologies. Based on detailed petrographic and mineral chemical data we show that the various units crystallized under markedly different oxygen fugacity and silica activity conditions and demonstrate how these parameters influence both the phase assemblage and the detailed chemical evolution of the fractionating phases. Nepheline, olivine–clinopyroxene and hornblende–plagioclase thermometry indicate equilibration temperatures ≥800°C for all major rock types. Highly oxidized conditions (close to the hematite–magnetite buffer) are characteristic of the garnet-rich pyroxenites, ultrapotassic glimmerites and associated olivine-shonkinites. The parental magmas to these rocks evolved from low initial a SiO2 values of 0·1 to values of 0·5–0·8 during nepheline and alkali feldspar saturation. In contrast, the monzonitic rocks evolved from initially high a SiO2 values (up to 0·75) down to about 0·1 at intermediate values of oxygen fugacity (ΔFMQ = +2–5 to −1, where FMQ is the fayalite–magnetite–quartz buffer). For nepheline syenites and malignites, more reduced conditions (ΔFMQ = −2) and intermediate a SiO2 values (between 0·25 and 0·5) dominate. We conclude that fractional crystallization is not a likely mechanism to explain the large variety of lithologies present in the Tamazeght complex. It is more probable that successive melting of a compositionally heterogeneous mantle source region gave rise to several melt batches with distinct chemical and physico-chemical characteristics. Low-degree melts from a K-phase-bearing mantle domain resulted in the formation of ultrapotassic glimmerites, whereas garnet-rich pyroxenites and olivine-shonkinites may have originated from hybrid melts and partly from a pyroxene-dominated source. Less alkaline lithologies such as monzonites potentially reflect larger degrees of melting and the increased importance of a basaltic component, whereas nepheline syenites and malignites may be explained by lower degrees of melting and a more alkaline character for the parental melt of these rocks.

Fluid interaction and element mobility in the development of ultramylonites

Abstract: Shear zones are often characterized by the presence of mylonitic fabrics. The textural development of such fabrics is enhanced by the presence of a fluid phase. In a single specimen of rock from the Brevard fault zone in North Carolina, we can demonstrate the development of ultramylonite domain by focused fluid flow. The ultramylonite interface retains a sharp textural and chemical discontinuity; this suggests that solute transport was dominantly parallel to the tectonic layering. Major chemical changes between the ultramylonite zone and the protolith include losses of Si(>2, Na 2 0, and K 2 0 and gain of CaO, FeO, and HjO in the high-strain domain. INTRODUCTION During the tectono-thermal evolution of mountain belts, zones of high strain are often recorded by the development of mylonites. Although mylonites and subcategories within mylonites (Sibson, 1977) can be generated by numerous well-documented physical processes, their chemi-cal and isotopic evolution is much more complex. Mechanical processes often associated with the development of high-strain zones include micro-fracturing, dilatancy, grain-boundary sliding, and dislocation glide. These properties are strongly dependent on the modal mineralogy (bulk compo-sition), heterogeneity of the rock, and the temperature and pressure con-ditions during deformation. Such variations in the mechanics of deforma-tion have led to the more common classification of shear zones as either ductile or brittle (Sibson, 1977; Ramsay, 1980). However, most shear zones are characterized by the availability of fluids that cause a drastic change in the style of deformation by altering the behavior and assem-blage of minerals (hydration reactions) and the rates of chemical and mechanical processes during progressive deformation. The presence of a fluid phase during deformation enhances solution transport, microfractur-ing, and recrystallization Suc.h chemically active zones generate mineral assemblages that record the metamorphic history of the mylonites, al-though it is not unusual to find minerals that persist through such an event. These relict minerals, if recognized, can identify equilibrium vs. nonequilibrium assemblages observed in shear zones. Clearly, the pres-ence of relict minerals would have a substantial impact on the trace-element and isotopic signature of the rock. To more carefully document the behavior of elements in a strain zone that has abundant fluid, we have chosen to study a transect across the Brevard fault zone, near Rosman, North Carolina. Within this tran-sect, one particular outcrop provided a single large specimen that over a distance of-25 cm showed the entire strain-gradient fabric from proto-mylonite to ultramylonite (see Sibson, 1977, for mylonite series) ob-served by us as discontinuous patches in the longer (15-km) transect. We report petrographic, textural, and chemical data from this block and pro-vide a physical model to explain these data.