Shengwei Wu

Mafic microgranular enclaves (MMEs) are widespread in granitic plutons and provide valuable insights into mush–magma mixing processes in crustal magma reservoirs. In this study, we characterize chemical zoning and Sr isotopic compositions of plagioclase in the MMEs, gabbro and host monzogranite from the Late Triassic Xiuyan pluton in East China, to constrain the origin of MMEs and the role of crystal mushes in magma mixing. The MMEs in the Xiuyan pluton are angular and range from centimeters to several meters in size. They exhibit sharp contacts with the host monzogranite and show diverse disequilibrium textures. Plagioclase in MMEs occurs as fine-grained antecryst with normal zoning (An_46–66 in the core and An_17–29 in the rim). The cores are commonly characterized by coarse sieve textures, patchy zoning, and resorption surfaces at core–rim boundaries. In situ Sr isotopic compositions show subtle but systematic core–rim variations, with (⁸⁷Sr/⁸⁶Sr)_i increasing slightly from cores (~0.70639) to rims (~0.70664), and rim values overlapping the whole-rock (⁸⁷Sr/⁸⁶Sr)_i of MMEs. These features suggest that the rim was crystallized from locally hybridized melts produced by interaction between interstitial melts in a basaltic mush and granitic magma. Plagioclase in the gabbro occurs as medium-grained phenocryst with normal zoning (An_46–65 in the core and An_18–27 in the rim) but shows nearly homogeneous (⁸⁷Sr/⁸⁶Sr)_i across individual grains (0.70612–0.70637), comparable to whole-rock gabbro values of 0.70623. The plagioclase cores in gabbro also show coarse sieve texture and patchy zoning with the resorption surface in the margin of the core and rim. We interpret the sieve textures in plagioclase cores from both MMEs and gabbro to record partial dissolution during rapid ascent and decompression of an initially H₂O-undersaturated, crystal-bearing basaltic magma, during which increased effective water activity reduced plagioclase stability prior to the growth of the rim. Plagioclase in the host monzogranite is medium- to coarse-grained, compositionally homogeneous, and characterized by low An contents (An_12–24) and elevated (⁸⁷Sr/⁸⁶Sr)_i of ~0.70828. We propose that MMEs in the Xiuyan pluton formed when semi-consolidated mafic mush was mechanically disaggregated into angular fragments and subsequently entrained into coexisting granitic melt. This study reveals that MMEs formed by mechanical disaggregation of a semi-consolidated mafic mush into angular fragments, followed by their entrainment into the granitic melts.

The geochemistry of rare earth elements and yttrium (REY) in phosphorite has been widely studied. However, the effect of organic matter on REY enrichment has not been well determined. We utilized paired inorganic (δ¹³C_carb) and organic (δ¹³C_carb) carbon isotopes, total organic carbon (TOC), and REY content (∑REY) of the Zhijin Motianchong (MTC) phosphorite and compared them with those of Meishucun (MSC) phosphorite to reveal the effect of organic matter on REY. The δ¹³C_carb of the MTC area (≈0‰) is heavier than that of the MSC area (−5.23‰ to −1.13‰), whereas δ¹³C_org is lighter (−33.85‰ to −26.34‰) in MTC than in MSC (−32.95‰ to −25.50‰). Decoupled δ¹³C_carb and δ¹³C_org in MTC indicate the contribution of chemoautotrophic organisms or methanotrophic bacteria. Compared to the MSC phosphorite, the MTC phosphorite has higher ∑REY and TOC, and these parameters have a positive relationship. MTC phosphorite has REY patterns resembling those of contemporary organic matter. Furthermore, dolomite cement has a higher ∑REY than dolomite in the phosphorus-bearing dolostone. Additionally, pyrites are located on the surface of fluorapatite in the Zhijin phosphorites. It is reasonable to suggest that the REY was released into the pore water owing to the anaerobic oxidation of organic matter at the interface between seawater and sediment, resulting in the REY enrichment of Zhijin phosphorites.

The rare earth elements and yttrium (REY)-enriched phosphorites in the Zhijin region, southwest China, have attracted much attention, yet its origin has not been sufficiently addressed. The geology, mineralogy and geochemistry of samples from four sections of the early Cambrian age from the Zhijin region were studied to attain the redox condition and origin of REY. Data from in situ analysis show that REY are mainly contained in francolite, and the two types of francolite (bioclastic and granular) have no distinct difference in REY content (ΣREY). A Ce anomaly indicates an oxic condition in primary seawater. An Eu anomaly and ratios of redox sensitive elements (RSEs) indicate a fluctuating redox condition during phosphorite deposition. Shale-normalized REY patterns of whole rock and francolite show a hat-shaped pattern, indicating the effect of diagenetic processes. The Y/Ho ratio of the selected sections ranges from 38.13 to 61.93, and together with the Y anomaly, La_N/Nd_N ratio, La_N/Sm_N ratio, and La_N/Yb_N ratio, this indicates a seawater origin. This is supported by the Ce/Ce* of the phosphorite that ranges from 0.32 to 0.52 and the Eu/Eu* that ranges from 0.88 to 1.82, which is similar to the characteristics of seawater and deep-sea mud. We also propose a minor contribution of terrigenous debris and influence of diagenesis and hydrothermal processes.