St. Paul Island is the youngest volcanic center in the Bearing Sea basalt province. We have undertaken a field, petrographic, and geochemical study of select St. Paul volcanic rocks in order to better understand their differentiation; specifically, to test the hypothesis that magmas erupted from individual Bering Sea basaltic volcanoes are not related by shallow-level processes such as crystal fractionation. Petrographically, all of the St. Paul volcanic rocks are olivine-, plagioclase-, and clinopyroxene-phyric. Textural features and modal contents of olivine phenocrysts, however, vary widely throughout the spectrum of basalt compositions. Although differing in size and abundance, olivine phenocrysts in all rock compositions are euhedral and commonly skeletal, suggesting rapid growth during ascent or eruptive quenching. None, however, display reaction textures with surrounding groundmass liquid. Compositionally, the St. Paul volcanic rocks are basalts and tephritic basalts and all have high contents of normative nepheline (8% to 16%). Concentrations of many major and incompatible trace elements display no clear correlations with bulk-rock SiO2 and MgO contents or modal abundances of phenocrysts, suggesting that much of the compositional diversity of these magmas reflects variable mantle sources and degrees of partial melting. Similarly, chondrite-normalized REE patterns show variable degrees of light REE enrichment (Lan=70–90) that do not correlate with bulk-rock mg-numbers. In contrast, concentrations of compatible trace elements (Ni, Cr, and Co) are positively correlated with MgO contents and modal percentages of olivine phenocrysts. Maximum forsterite contents of olivine phenocryst cores in most St. Paul rocks decrease with decreasing bulk-rock mg-number and are similar to the calculated equilibrium range. This is evidence that the high mg-numbers are magmatic and do not result from olivine accumulation. Instead, major and compatible trace element mass balance calculations support derivation of the low mg-number lavas from the high mg-number lavas mainly by olivine fractionation, which, in turn, implies that St. Paul magmas may have temporarily resided in crustal magma chambers prior to eruption. 相似文献
Silicate-melt inclusions in igneous rocks provide important information on the composition and evolution of magmatic systems. Such inclusions represent accidentally trapped silicate melt (±immiscible H2O and/or CO2 fluids) that allow one to follow the evolution of magmas through snapshots, corresponding to specific evolution steps. This information is available on condition that they remained isolated from the enclosing magma after their entrapment. The following steps of investigation are discussed: (a) detailed petrographic studies to characterise silicate-melt inclusion primary characters and posttrapping evolution, including melt crystallisation; (b) high temperature studies to rehomogenise the inclusion content and select chemically representative inclusions: chemical compositions should be compared to relevant phase diagrams.
Silicate-melt inclusion studies allow us to concentrate on specific topics; inclusion studies in early crystallising phases allow the characterisation of primary magmas, while in more differentiated rocks, they unravel the subsequent chemical evolution. The distribution of volatile species (i.e., H2O, CO2, S, Cl) in inclusion glass can provide information on the degassing processes and on recycling of subducted material. In intrusive rocks, silicate melt inclusions may preserve direct evidence of magmatic stage evolution (e.g., immiscibility phenomena). Melt inclusions in mantle xenoliths indicate that high-silica melts can coexist with mantle peridotites and give information on the presence of carbonate melt within the upper mantle. Thus, combining silicate-melt inclusion data with conventional petrological and geochemical information and experimental petrology can increase our ability to model magmatic processes. 相似文献
The coal of the Miocene Bukit Asam deposit in south Sumatra is mostly sub-bituminous in rank, consistent with regional trends due to burial processes. However, effects associated with Plio–Pleistocene igneous intrusions have produced coal with vitrinite reflectance up to at least 4.17% (anthracite) in different parts of the deposit. The un-metamorphosed to slightly metamorphosed coals, with Rvmax values of 0.45–0.65%, contain a mineral assemblage made up almost entirely of well-ordered kaolinite and quartz. The more strongly heat-affected coals, with Rvmax values of more than 1.0%, are dominated by irregularly and regularly interstratified illite/smectite, poorly crystallized kaolinite and paragonite (Na mica), with chlorite in some of the anthracite materials. Kaolinite is abundant in the partings of the lower-rank coals, but is absent from the partings in the higher-rank areas, even at similar horizons in the same coal seam. Regularly interstratified illite/smectite, which is totally absent from the partings in the lower-rank coals, dominates the mineralogy in the partings associated with the higher-rank coal beds. A number of reactions involving the alteration of silicate minerals appear to have occurred in both the coal and the associated non-coal lithologies during the thermal metamorphism generated by the intrusions. The most prominent involve the disappearance of kaolinite, the appearance of irregularly interstratified illite/smectite, and the formation of regular I/S, paragonite and chlorite. Although regular I/S is identified in all of the non-coal partings associated with the higher-rank coals, illite/smectite with an ordered structure is only recognised in the coal samples collected from near the bases of the seams. The I/S in the coal samples adjacent to the floor of the highest rank seam also appears to have a greater proportion of illitic components. The availability of sodium and other non-mineral inorganic elements in the original coal to interact with the kaolinite, under different thermal and geochemical conditions, appears to be the significant factor in the formation of these new minerals, and distinguishes the mineralogical changes at Bukit Asam from those developed more generally with rank increases due to burial, and from the effects of intrusions into coals that were already at higher rank levels. 相似文献
Mafic and felsic rocks units of the Musgrave Province originally attributed to the c. 1075 Ma Giles Event of the greater Warakurna Large Igneous Province (LIP) are shown to be part of a complex sequence of magmatic and tectonic events punctuated over a span of at least 50 m.y. New geochronology and mapping resolve a sequence of at least 10 magmatic pulses with hiati of up to 10 m.y. consistent with a long-lived intracontinental rift setting. This rift, here named the Ngaanyatjarra Rift, features giant layered mafic-ultramafic Giles intrusions cut by a 10 km wide mafic-felsic magmatic shear zone. The latter is temporally related to the Warakurna LIP, however it is not clear that the Giles intrusions actually form part of the Warakurna LIP. Macroscopic folding and the formation of the large synmagmatic transpressional shear zone attest to synmagmatic basin inversion in the early stages of the rift. The extensive mafic to felsic volcanic rocks of the Tollu Group (traditionally grouped with the Giles Event) were emplaced 25–50 m.y. later than the c. 1075 Ma Warakurna LIP. 相似文献