Metasomatism and magmatic assimilation at a gabbro-limestone contact, Christmas Mountains, Big Bend region, Texas

A composite stock of alkaline gabbro and syenite is intrusive into limestone of the Del Carmen, Sue Peake and Santa Elena Formations at the northwest end of the Christmas Mountains. There is abundant evidence of solution of wallrock by magma but nowhere are gabbro and limestone in direct contact. The sequence of lithologies developed across the intrusive contact and across xenoliths is gabbro, pyroxenite, calc-silicate skarn, marble. Pyroxenite is made up of euhedral crystals of titanaugite and sphene in a leucocratic matrix of nepheline, Wollastonite and alkali feldspar. The uneven modal distribution of phases in pyroxenite and the occurrence' of nepheline syenite dikes, intrusive into pyroxenite and skarn, suggest that pyroxenite represents an accumulation of clinopyroxene "cemented" together by late-solidifying residual magma of nepheline syenite composition. Assimilation of limestone by gabbroic magma involves reactions between calcite and magma and/or crystals in equilibrium with magma and crystallization of phases in which the magma is saturated, to supply energy for the solution reaction. Gabbroic magma was saturated with plagioclase and clinopyroxene at the time of emplacement. The textural and mineralogic features of pyroxenite can be produced by the reaction 2( 1-X) CALCITE + AN_XAB_l-X = (1-X) NEPHELINE+ 2(1-X) WOLLASTONITE+ X ANORTHITE+ 2(1-X) CO₂. Plagioclase in pyroxenite has corroded margins and is rimmed by nepheline, suggestive of resorption by magma. Anorthite and wollastonite enter solid solution in titanaugite. For each mole of calcite dissolved, approximately one mole of clinopyroxene was crystallized. Thus the amount of limestone that may be assimilated is limited by the concentration of potential clinopyroxene in the magma. Wollastonite appears as a phase when magma has been depleted in iron and magnesium by crystallization of titanaugite. The predominance of mafic and ultramafic compositions among contaminated rocks and their restriction to a narrow zone along the intrusive contact provides little evidence for the generation of a significant volume of desilicated magma as a result of limestone assimilation.

Within 60 m of the intrusive contact with the gabbro, nodular chert in the Santa Elena Limestone reacted with the enveloping marble to form spherical nodules of high-temperature calc-silicate minerals. The phases wollastonite, rankinite, spurrite, tilleyite and calcite, form a series of sharply-bounded, concentric monomineralic and two-phase shells which record a step-wise decrease in silica content from the core of a nodule to its rim. Mineral zones in the nodules vary 'with distance from the gabbro as follows:

0-5 m CALCITE + SPURRITE + RANKINITE + WOLLASTONITE
5-16 m CALCITE + TILLEYITE ± SPURRITE + RANKINITE + WOLLASTONITE
16-31 m CALCITE + TILLEYITE + WOLLASTONITE
31-60 m CALCITE + WOLLASTONITE
60-plus CALCITE + QUARTZ

The mineral of a one-phase zone is compatible with the phases bounding it on either side but these phases are incompatible in the same volume of P-T-X_CO2.

Growth of a monomineralio zone is initiated by reaction between minerals of adjacent one-phase zones which become unstable with rising temperature to form a thin layer of a new single phase that separates the reactants and is compatible with both of them. Because the mineral of the new zone is in equilibrium with the phases at both of its contacts, gradients in the chemical potentials of the exchangeable components are established across it. Although zone boundaries mark discontinuities in the gradients of bulk composition, two-phase equilibria at the contacts demonstrate that the chemical potentials are continuous. Hence, Ca, Si and CO₂ were redistributed in the growing nodule by diffusion. A monomineralic zone grows at the expense of an adjacent zone by reaction between diffusing components and the mineral of the adjacent zone. Equilibria between two phases at zone boundaries buffers the chemical potentials of the diffusing species. Thus, within a monomineralic zone, the chemical potentials of the diffusing components are controlled external to the local assemblage by the two-phase equilibria at the zone boundaries.

Mineralogically zoned calc-silicate skarn occurs as a narrow band that separates pyroxenite and marble along the intrusive contact and forms a rim on marble xenoliths in gabbro. Skarn consists of melilite or idocrase pseudomorphs of melili te, one or two . stoichiometric calcsilicate phases and accessory Ti-Zr garnet, perovskite and magnetite. The sequence of mineral zones from pyroxenite to marble, defined by a characteristic calc-silicate, is wollastonite, rankinite, spurrite, calcite. Mineral assemblages of adjacent skarn zones are compatible and the set of zones in a skarn band defines a facies type, indicating that the different mineral assemblages represent different bulk compositions recrystallized under identical conditions. The number of phases in each zone is less than the number that might be expected to result from metamorphism of a general bulk composition under conditions of equilibrium, trivariant in P, T and u_CO2. The "special" bulk composition of each zone is controlled by reaction between phases of the zones bounding it on either side. The continuity of the gradients of composition of melilite and garnet solid solutions across the skarn is consistent with the local equilibrium hypothesis and verifies that diffusion was the mechanism of mass transport. The formula proportions of Ti and Zr in garnet from skarn vary antithetically with that of Si Which systematically decreases from pyroxenite to marble. The chemical potential of Si in each skarn zone was controlled by the coexisting stoichiometric calc-silicate phases in the assemblage. Thus the formula proportion of Si in garnet is a direct measure of the chemical potential of Si from point to point in skarn. Reaction between gabbroic magma saturated with plagioclase and clinopyroxene produced nepheline pyroxenite and melilite-wollastonite skarn. The calcsilicate zones result from reaction between calcite and wollastonite to form spurrite and rankinite.

Ecological study of chalk and limestone streams. Annual Report October 1974 to September 1975

Few detailed studies have been made on the ecology of the chalk streams. A complex community of plants and animals is present and much more information is required to achieve an understanding of the requirements and interactions of all the species. It is important that the rivers affected by this scheme should be studied and kept under continued observation so that any effects produced by the scheme can be detected. The report gives a brief synopsis of work carried out during the second year of a four year ecological study sponsored jointly by the Thames Water Authority and the Central Water Planning Unit. It assumes some familiarity with the investigations carried out on the River Lambourn during the preceding three years which was sponsored jointly by the Thames Conservancy and Water Resources Board (immediate predecessors of the present sponsoring organisations). (PDF contains 31 pages)

Ecological study of chalk and limestone stream. Annual report October 1975 to September 1976

Few detailed studies have been made on the ecology of the chalk streams. A complex community of plants and animals is present and much more information is required to achieve an understanding of the requirements and interactions of all the species. It is important that the rivers affected by this scheme should be studied and kept under continued observation so that any effects produced by the scheme can be detected. The report gives a brief synopsis of work carried out during the third year of a four year ecological study sponsored jointly by the Thames Water Authority and the Central Water Planning Unit. It assumes . It assumes some familiarity with the investigations carried out on the River Lambourn during the preceding three years which was sponsored jointly by the Thames Conservancy and Water Resources Board (immediate predecessors of the present sponsoring organisations). (PDF contains 35 pages)

Reavaliação deposicional da Bacia de São José de Itaboraí com base em dados geológicos e geofísicos

A Bacia de São José de Itaboraí está localizada no Município de Itaboraí, no Estado do Rio de Janeiro. Ela foi descoberta em 1928, pelo Engenheiro Carlos Euler, que após analisar um suposto caulim encontrado na Fazenda São José pelo seu então proprietário, Sr. Ernesto Coube, verificou que se tratava de calcário. Os Professores Rui Lima e Silva e Othon H. Leonardos, enviados ao local para estudos, encontraram uma grande quantidade de fósseis de gastrópodes continentais, despertando o interesse científico pela região. Os estudos preliminares de campo e análises químicas evidenciaram boas perspectivas de exploração do calcário para a fabricação de cimento do tipo Portland. Por mais de 50 anos, a Companhia Nacional de Cimento Portland Mauá (CNCPM) explorou a pedreira. Desde sua descoberta, a Bacia de São José, paralelamente às atividades de mineração, foi objeto de pesquisas científicas realizadas por geólogos, paleontólogos e arqueólogos. No início da década de 80, a Cia. de Cimento Mauá decidiu abandonar a área em função do esgotamento econômico da reserva de minério. Com a retirada das bombas que impediam a inundação da pedreira, formou-se uma lagoa que passou a impedir o livre acesso aos afloramentos. Desde então as pesquisas sobre a Bacia ficaram concentradas aos materiais coletados no período de exploração de calcário. Material esse distribuído no Museu Nacional (MN), Departamento Nacional da Produção Mineral (DNPM), Instituto de Geociências da UFRJ, entre outros. Em 1990, a área que pertencia a CNCPM foi desapropriada por pressão da comunidade científica. A mesma passou a pertencer ao Município de Itaboraí, que criou o Parque Paleontológico de São José de Itaboraí, por meio da Lei 1.346, de 12 de dezembro de 1995. O objetivo desse trabalho foi gerar novos dados através do método geofísico conhecido como magnetometria. Para isso foram realizados levantamentos de campo utilizando um magnetômetro portátil e GPS, foram analisados e corrigidos dados utilizando softwares específicos, elaborados modelos e criados perfis a partir de descrições de testemunhos de sondagem. Os resultados obtidos visam possibilitar uma nova interpretação da geologia e da estratigrafia da bacia, dando condições para que se possa ter uma atualização dos conhecimentos relacionados à região, após quase meio século de atividade mineradora.

西秦岭晚古生代弧前盆地沉积与成矿作用

The Western Qinling Orogenie belt in the Taibai-Fengxian and Xihe-Lixian areas can be subdivided into three units structurally from north to south, which are the island-arc, forearc basin and accretionary wedge, respectively. The forearc basin developed in the Late Paleozoic mainly controls sedimentation and some larger lead-zinc and gold deposits in the western Qinling. Stratigraphically, the island arc is dissected into the Liziyuan Group, the Danfeng Group and the Luohansi Group. The metavolcanic rocks include basic, intermediate and acidic rocks, and their geochemistry demonstrates that these igneous rocks generated in an island arc. Where, the basalts are subalkaline series charactered by low-medium potassium, with enriched LREE, negative Eu anomaly, and positive Nd anomaly. Cr-content of volcanic rocks is 2-3 times higher than that of island arc tholeiite all over the world. In addition, the lightly metamorphosed accretionary wedge in the areas of Huixian, Chengxian, Liuba and Shiqun is dominated by terrigenous sediments with carbonatite, chert, mafic and volcanic rocks. The age of the wedge is the Late Palaeozoic to the Trassic, while previous work suggested that it is the Silurian. The Upper Paleozoic between the island arc belt and accretionary wedge are mainly the sediments filled in the fore arc basin. The fillings in the forearc basin were subdivided into the Dacaiotan Group, the Tieshan Group, the Shujiaba Group and the Xihanshui Group, previously. They outcropped along the southern margins of the Liziyuan Group. The Dacaotan Group, the Upper Devonian, is close to the island arc complex, and composed of a suite of red and gray-green thick and coarse terrestrial elastics. The Shujiaba Group, the Mid-Upper Devonian, is located in the middle of the basin, is mainly fine-grained elastics with a few intercalations of limestone. The Xihanshui Group, which distributes in the southern of the basin, is mainly slates, phyllites and sandstones with carbonatite and reef blocks. The Tieshan Group, the Upper Devonian, just outcrops in the southwest of the basin, is carbonatite and clastic rocks, and deposited in the shallow -sea environment. The faults in the basin are mainly NW trend. The sedimentary characteristics, slump folds, biological assemblages in both sides of and within those faults demonstrate that they were syn-sedimentary faults with multi-period activities. They separated the forearc basin into several sub-basins, which imbricate in the background of a forearc basin with sedimentary characteristics of the piggyback basin. The deep hydrothermal fluid erupted along the syn-sedimentary faults, supported nutrition and energy for the reef, and resulted in hydrothermal-sedimentary rocks, reef and lead-zinc deposits along these faults. The sedimentary facies in the basin varies from the continental slope alluvial fan, to shallow-sea reef facies, and then to deep-water from north to south, which implies that there was a continental slope in the Devonian in the west Qinling. The strata overlap to north and to east respectively. Additionally, the coeval sedimentary facies in north and south are significantly different. The elastics become more and more coarser to north in the basin as well as upward coarsing. These features indicate prograding fillings followed by overlaps of the different fans underwater. The paleocurrent analyses show that the forearc basin is composed of thrust-ramp-basins and deep-water basins. The provenance of the fillings in the basin is the island arc in the north. The lead-zinc deposits were synchronous with the Xihanshui Group in the early stage of development of the forearc basin. They were strongly constrained by syn-sedimentary faults and then modified by the hydrothermal fluids. The gold deposits distributed in the north of the basin resulted from the tectonic activities and magmatism in the later stage of the basin evolution, and occurred at the top of the lead-zinc deposits spatially. The scales of lead-zinc deposits in the south of the basin are larger than that of the gold-deposits. The Pb-Zn deposits in the west of the basin are larger than those in the east, while the Gold deposits in the west of the basin are smaller than those in the east. Mineralizing ages of these deposits become younger and younger to west.