Progress in Continuous Fractional Crystallization

Mineralogical constraints on pegmatite genesis and rare metal mineralization in the Mufushan batholith, South China

Integration of geochemical, mineralogical, isotopic, and geochronological data with geodynamic considerations suggests that the Variscan granites in the Erzgebirge-Slavkovský les domain originated from repeated melting events and were emplaced over a period of about 40 Ma (330–290 Ma). Several lines of evidence exist supporting the idea that Erzgebirge granites assigned to different types (biotite granites, two-mica granites, strongly peraluminous P-rich Li-mica granites, and slightly peraluminous P-poor granites) are in most cases not genetically related via continuous fractional crystallization from a common magmatic reservoir. The genesis of the Slavkovsky les granites, however, might be discussed in terms of an uninterrupted fractionation series. Geological models of Sn-W deposits based upon geochemical and structural results imply that the main ore depositional events followed immediately the emplacement and solidification processes of melt via fluid-melt immiscibility, breccia-pipe formation and/or pervasive rock-fluid interactions.

ABSTRACTThe south hinge of the Torres Syncline in southernmost Brazil hosts a volcanic succession of pahoehoe and rubbly Gramado-type lavas belonging to the ~132 Ma Paraná–Etendeka Igneous Province. We evaluate the geochemical and Sr–Nd–Pb isotopic variations using local-scale stratigraphy in order to discuss the petrogenesis of lava flows in a single magma type and to estimate the role of crustal contamination and the potential contaminants involved. The geochemical and isotopic variations along the lava pile are not systematic, implying that the magma chamber could have undergone successive replenishments of basaltic magma. The process of crustal assimilation explains the high and widespread initial Sr isotopic ratios at 0.707798–0.715751 and the very low εNd at −8.36 to −5.41, with associated Pb isotopic variations (18.42 < 206Pb/204Pb < 18.86; 15.65 < 207Pb/204Pb < 15.71; 38.62 < 208Pb/204Pb < 39.37). The magmatic evolution of the SCSH and LJ lava flows begins with the storage of mafic liquids during a short period in the shallow-level magma chamber, which allowed the magma ascent with composition of olivine basalts. The continuous fractional crystallization within the magma chamber coupled with variable assimilation degrees of distinct contaminants with Palaeoproterozoic and Neoproterozoic ages, in addition to significant contribution of magma recharge, led to magma ascent with basaltic andesite composition that displays at surface a simple pahoehoe morphology. The continuous magma recharge in the magma chamber coupled with higher assimilation degree allowed the formation of basaltic andesite lavas with more contaminated isotopic signatures that exhibit rubbly morphology at the surface. Differentiation process of liquids coupled with the highest assimilation degrees of distinct contaminants during longer time in a shallow-level magma chamber, which is distinct from that where SCSH and LJ magmas have been stored, led to the formation of andesites of the Morro da Cruz section that exhibit the most contaminated isotopic signatures.

The possibilities of separation and purification by crystallization are determined by the solid-liquid phase equilibria. Two well-known types of equilibrium diagrams are those belonging to the eutectic and solid solution systems.

The Kop ophiolite in NE Turkey is a fragment of Neo‐Tethyan forearc. It can be mainly divided into a paleo‐Moho transition zone (MTZ) in the North and a harzburgitic mantle sequence in the South. Dunites are predominant in the MTZ of the Kop ophiolite, and they are locally interlayered with chromitites and enclose minor bodies of harzburgites near the petrological Moho boundary. Large Fe isotopic variations were observed for magnesiochromite (−0.14‰ to 0.06‰) and olivine (−0.12‰ to 0.14‰) from the MTZ chromitites, dunites and harzburgites. In individual dunite samples, magnesiochromite usually has lighter Fe isotopic compositions than olivine, which was probably caused by subsolidus Mg‐Fe exchange between the two mineral phases. Both magnesiochromite and olivine display an increasing trend of δ56Fe along a profile from chromitite to dunite. This trend reflects continuous fractional crystallization in a magma chamber, which resulted in heavier Fe isotopes concentrated in the evolved magmas. In each cumulative cycle of chromitite and dunite, dunite was formed from relatively evolved melts after massive precipitation of magnesiochromite. Mixing of more primitive and evolved melts in the magma chamber was a potential mechanism for triggering the crystallization of magnesiochromite, generating chromitite layers in the cumulate pile. Before mixing happened, the primitive melts had reacted with mantle harzburgites during their ascendance; whereas the evolved melts may lie on the olivine‐chromite cotectic near the liquidus field of pyroxene. Variable degrees of magma mixing and differentiation are expected to generate melts with different δ56Fe values, accounting for the Fe isotopic variations of the Kop MTZ.

ABSTRACTTemporal and spatial variations in the Nd isotopic compositions of Tertiary caldera-forming rhyolite tuffs, and Cretaceous and Tertiary granites of the western U.S.A. are used as a basis for a model that accounts for the observed proportions of crustal versus mantle contributions to silicic magmas in terms of two parameters: the ambient crustal temperature and the rate of supply of basaltic magma from the mantle. The crustal contribution to silicic igneous rocks is measured in terms of the Neodymium Crustal Index (NCI). The relationships between crustal temperature, basalt supply and NCI are quantified using a model of a magma chamber undergoing continuous recharge, wall-rock assimilation and fractional crystallisation. From the model, a critical value of the ratio of basalt recharge-to-assimilation, (r/a)c, is deduced, which increases with decreasing crustal temperature. The r/a value must exceed (r/a)cto allow the volume of differentiated magma to increase, a prerequisite for developing large volumes of silicic magma. Strongly peraluminous (or S-type) magmas (NCI = 0·8–1), form under conditions of high crustal temperature and low basalt supply. Metaluminous or I-type granites form over a wide range of conditions (NCI = 0·1–1), generally where basalt supply is substantial. In individual long-lived volcanic centres, the large-volume high-silica ignimbrites are associated with the highest r/a and lowest NCI values, indicating that these magmas are typically differentiates of mantle-derived basaltic parents.