Batch melting, fractional melting, continuous melting and two-porosity melting models have been used widely in geochemical studies of trace element fractionation during mantle melting. These simple melting models were developed for melting an homogeneous mantle source. Here we revisit and further develop these melting models in the context of decompression melting of a two-lithology mantle. Each lithology has its own source composition and melting parameters. During decompression melting, melt and solid flow vertically in the melting column. Part of the melt produced in one lithology is transferred to the other lithology at a prescribed rate. We use a set of conservation equations to solve for melt and solid mass fluxes, extent of melting and concentrations of a trace element in interstitial melt and aggregated melt in each lithology and mixed-column melt between the two lithologies. We uncover conditions under which batch melting, fractional melting, continuous melting and two-porosity melting models are realized during decompression melting through four case studies. We show that porosity in the continuous melting model varies along the melting column during decompression melting, contrary to what was assumed in its original development. We unify the batch melting, fractional melting, continuous melting and two-porosity melting models through a two-lithology melting model for decompression melting in a two-lithology mantle column. We discuss basic features of the two-lithology melting model through worked examples. We show that it is possible to produce partial and well-mixed melts with a range of REE patterns, from LREE depleted to LREE enriched, similar to those observed in mid-ocean ridge basalts by decompression melting of a two-lithology mantle.

Abstract China presently comprises several independent tectonic palaeoplates or terranes and parts of other blocks, which have been assembled over geological time. In the Ordovician, these blocks included South China, North China, Tarim, Qaidam, Junggar, Qiangtang-Qamdo, Lhasa and partially Himalaya, Sibumasu and Indochina, as well as the Altay-Xing'an and Songpan-Garze fold belts, which were discrete but near-adjacent. Twelve stratigraphic megaregions bounded by tectonic sutures or major fault zones can be recognized. Some of them are further differentiated into several regions according to the lithological and biotic facies or distinct stratigraphic sequences. Here, the palaeontologic features and biostratigraphic framework of these stratigraphic megaregions and regions are summarized. The unified biostratigraphic framework presented herein is supported by 33 graptolite biozones and 27 conodont biozones, together with supplementary biozones, communities or associations of brachiopods, trilobites, cephalopods, chitinozoans, acritarchs and radiolarians. With constraints of integrative chronostratigraphy, biostratigraphy, chemostratigraphy, cyclostratigraphy and magnetostratigraphy, along with some geochronologic data, our understanding of the temporal and spatial distribution of the Ordovician lithostratigraphic units on these major blocks has been significantly advanced. Vast amounts of new data accumulated in recent decades also constrain the major Ordovician geological and biotic events evident in China, such as marine anoxia, faunal turnovers and tectonic orogenies.

Abstract Two remarkable events in the history of life on the Earth occur during the Ordovician Period (486.9–443.1 Ma). The first is an exceptionally rapid and sustained radiation of marine life known as the ‘Great Ordovician Biodiversification Event’ (GOBE), and the second is a catastrophic Late Ordovician mass extinction (LOME). Understanding the duration, rate and magnitude of these events requires an increasingly precise global correlation framework. In this chapter we review the major subdivisions of the Ordovician System, their Global Stratotype Section and Points, and the chronostratigraphic levels that define their bases. We also present a detailed set of correlation charts that illustrate the relationships between most of the regional graptolite, conodont and chitinozoan successions across the world.