Incompatible element

About: Incompatible element is a research topic. Over the lifetime, 2420 publications have been published within this topic receiving 154052 citations.

Multiple magmatic pulses of the Eastern Volcano-Plutonic Complex, Krušné hory/Erzgebirge batholith, and their phosphorus contents

Abstract: Granites of the eastern part of the Krusne hory/Erzgebirge (Czech Republic/Germany) outcrop in separate minor stocks formed by Late Palaeozoic CaO-rich biotite monzogranites of the Older Intrusive Complex (OIC) and syenogranites or alkali feldspar granites of the Younger Intrusive Complex (YIC). These, along with abundant rhyolites and granite porphyry dykes, constitute the Eastern Volcano-Plutonic Complex (EVPC). Using new analyses as well as older unpublished and literature data, we evidenced that these rocks display a wide spectrum of phosphorus contents ranging from very low (well below 0.1 wt% P2O5) to intermediate contents (from 0.1 to 0.4 wt% P2O5). No equivalent of the high-phosphorus fractionated granites with P2O5 > 0.4 wt% known from the western Krusne hory/Erzgebirge is present in the EVPC. We have demonstrated that the OIC granite bodies comprise a number of petrographic varieties and some of them represent individual magmatic pulses. Relatively elevated phosphorus contents in the evolved members of the Flaje granite massif may reflect their origin as individual magma batches from heterogeneous crustal sources. We observed the marginal Preisselberg granite enriched in P among the predominant P-poor YIC granites; this granite probably originated also as a separate magma batch. On the contrary, the rhyolitic ignimbrites, granite porphyries and YIC granites were derived from a younger, huge and long-standing magmatic system which presumably was formed and maintained as a consequence of significant thermal input from mantle-derived mafic magmas. These mafic magmas homogenized granitic partial melts from lower crustal sources and produced large volumes of highly evolved magma portions via fractional crystalliza- tion. The granite magmas with I-type or transitional I/S-type characteristics evolved towards the low-P2O5 compositions. This process was disturbed by episodic hybridization of the granite magmas with the mantle-derived melts or products of their fractionation at depth. The YIC granites probably represent residual, late-stage melts rich in volatiles, with strongly increased incompatible element abundances due to pro- longed crystal fractionation. Their composition was strongly affected by interaction with an aqueous fluid phase that changed the phosphorus distribution in only a minor way. Late magmatic and post-magmatic processes obscured the primary chemical characteristics namely in the Li-rich granites. This fact could be the reason for the absence of any correlation between the generally low P2O5 and highly variable alumina-saturation indices. The phosphorus contents thus reflect various aspects of rather complex genetic histories of Late Variscan igneous rocks in the EVPC, particularly within the lower and middle crust. In contrast with the role of phosphorus, the contents of lithium and fluorine are more dependent on subsolidus alterations of granites induced by hydrothermal fluids in late magmatic and post-magmatic stages at shallow crustal levels.

Magmatic recharge in continental flood basalts: Insights from the Chifeng igneous province in Inner Mongolia

Abstract: Eruptive sequences can be used as windows into the thermal and chemical evolution of magma chambers. We examined a continuous vertical section of the Baichahe basalt flow associated with the late Cenozoic Chifeng flood basalt in Inner Mongolia, North China. From oldest to youngest, MgO increases, K2O, light rare earths and other incompatible elements decrease, and Nb/La and radiogenic Pb isotopic ratios increase, all of which indicate increasing primitiveness and decreasing contribution of crustal contamination with time. The variable Pb isotope and incompatible element ratios require a component of crustal contamination, most likely of the lower crust (unradiogenic Pb, and low Ce/Pb), in the earliest lavas. Fractional crystallization can explain some of the elemental systematics, but alone cannot explain variable incompatible element ratios and Pb isotopes, nor the temporal trend to more primitive compositions. Crustal assimilation with or without fractional crystallization also cannot explain all the elemental systematics. We find instead that recharge by a primitive magma, in combination with fractional crystallization and decreasing rates of crustal assimilation, is needed to explain the observed geochemical systematics. Our observations suggest that the delivery of fresh basalt to the magma chamber must increase at rates faster than the crust can be assimilated or that the rates of crustal assimilation must decrease. However, progressive addition of primitive magma should heat up the crust and lead to more crustal assimilation. We suggest that during the initial stages of forming a magma chamber, the magma cools and develops an outer crystalline rind of mafic to ultramafic cumulates. This results in a thickening nonconvecting chemical boundary layer, which serves to insulate the magma chamber from further assimilation of crust and cooling, the latter resulting in the reduction of crystallization rates and the buffering of magma compositions at more primitive compositions. We show that certain segments of other large igneous provinces also display an evolution toward more primitive magmas with time, indicating that magmatic recharge may be a common feature of basaltic magma chambers.