Zr/hf fractionation in intraplate basaltic rocks: Carbonate metasomatism in the mantle source

http://www.mantleplumes.org/WebDocuments/Zhangjj09L.pdf

Geochemical evidence for interaction between oceanic crust and lithospheric mantle in the origin of Cenozoic continental basalts in east-central China

[magma volume (DRE): 24 ± 5 km3]. The main phase (ca. 95 vol.%) is represented by comenditic tephra deposited dominantly as widespread fallout blankets and proximal ignimbrites. The eruption column is estimated to have reached ca. 25 km and thus entered the stratosphere. A late phase (5 vol.%) is represented by trachyte emplaced chiefly as moderately welded ignimbrites. The comendites contain  ∼ 3, and the trachytes 10–20 vol.% phenocrysts, mainly anorthoclase, hedenbergite, and fayalite. Primary glassy melt inclusions with no signs of leakage were found only in phenocrysts in the comenditic tephra, whereas those in phenocrysts in the trachytes are devitrified. The comendite magma is interpreted to have been generated by fractional crystallization from a trachyte magma represented by melt inclusions in the phenocrysts in the comendite tephra. The mass of volatiles emitted to the atmosphere during the eruption was estimated using the petrologic method. The average H2O concentration of the comenditic matrix glass is 1.5 wt.% (probably largely secondary) and of the corresponding melt inclusions  ∼ 5.2 wt.%. Melt inclusions in feldspar and quartz present the highest halogen concentrations with a calculated average for chlorine of 4762 ppm and for fluorine of 4294 ppm. The comenditic matrix glasses are represented by a fluorine-rich (3992 ppm F) and fluorine-poor group (2431 ppm F), averaging 3853 ppm for chlorine. Only 20% of all sulfur analyses of the comenditic matrix glasses and melt inclusions are above the detection limit of  ≥ 250 ppm S. The difference between pre- and post-eruptive concentration of H2O is at least 3.7 ± 0.6 wt.% H2O taking into consideration re-hydration of the matrix glass and possible leakage of melt inclusions. The difference between pre- and post-eruptive concentrations of the halogens amounts to 909 ± 90 ppm Cl, and 1863 ± 280 ppm and 302 ± 40 ppm F. The difference for S was estimated based on the average of the maximum S concentrations in the melt inclusions (455 ppm S) and the detection limit, resulting in 205 ± 40 ppm S. The calculated mass of volatiles injected into the atmosphere, based on the erupted magma volume and volatile data, is 1796 ± 453 megatons for H2O, 45 ± 10 megatons for chlorine, 42 ± 11 megatons for fluorine, and 2 ± 0.6 megatons for sulfur. The 969 ± 20 AD eruption of Baitoushan Volcano, one of the largest eruptions of the past 2000 years, is thought to have had a substantial but possibly short-lived effect on climate.

Volatile emission during the eruption of Baitoushan Volcano (China/North Korea) ca. 969 AD

Cenozoic basalts from both sides of the Daxin’anling‐Taihang gravity lineament that separates the Sino‐Korean craton (SKC) into western and eastern parts have been studied. In the western SKC, magmas evolved from xenolith‐bearing alkali basalts of Late Eocene–Oligocene age to coexisting alkali and tholeiitic basalts of Late Miocene–Quaternary age. This change in basalt type is accompanied by a decrease in La/Yb and an increase in Yb content. Sr‐Nd isotopic ratios and relative abundances on incompatible elements are consistent with a prevailing asthenospheric origin, although lithospheric mantle may have also contributed to these basalts. This temporal variation in basalt geochemistry is interpreted as reflecting progressive lithospheric thinning in the western SKC during the Cenozoic. An opposite trend is observed for Cenozoic basalts from the eastern SKC, suggesting lithospheric thickening during this time period. This thickening was probably related to regional thermal decay following peak magm...

/pdf/contrasting-cenozoic-lithospheric-evolution-and-architecture-5660d40xt5.pdf

Contrasting Cenozoic Lithospheric Evolution and Architecture in the Western and Eastern Sino‐Korean Craton: Constraints from Geochemistry of Basalts and Mantle Xenoliths

Sensitive high resolution ion microprobe (SHRIMP) zircon U–Pb ages, geochemical and Sr-Nd-Pb isotopic data are reported for the gabbroic complex from Yinan (Shandong Province) with the aims of characterizing the nature of the Mesozoic mantle beneath the North China Craton. The Yinan gabbros contain alkali feldspar and biotite, and are characterized by moderate Mg#, high SiO2, low FeO and TiO2 contents and a strong enrichment of light rare earth elements [(La/Yb)n=11–50], but no Eu anomaly. They have low Nb/La (0.07–0.29), radiogenic 87Sr/86Sr (0.710) and unradiogenic eNd(t) (−15 to −13). These “crustal fingerprints” cannot be attributed to crustal contamination, given the lack of correlation between isotopic ratios and differentiation indices and the unreasonably high proportion of crustal contaminant (>20%) required in modeling. Instead, compositional similarities to contemporaneous basalts from nearby regions imply that the Yinan gabbros were not significantly affected by crystal cumulation. Isotopic data available for the Mesozoic mafic magmas reveal two distinct mantle domains beneath Shandong. While the EM1-like domain (with low 87Sr/86Sr) is confined to western Shandong, the mantle beneath eastern Shandong is dominated by EM2-type (with high 87Sr/86Sr) affinities. This aerial distinction suggests that the EM2-like signature of the Yinan gabbros may have been inherited from westerly-subducted Yangtze crust during the Triassic North China-South China collision. Emplacement of the Yinan gabbros (127 Ma) is likely affiliated with the widespread and protracted extension during the late Mesozoic in this region.

Early Cretaceous gabbroic complex from Yinan, Shandong Province: petrogenesis and mantle domains beneath the North China Craton

The Pliocene volcanic rocks of the Anhui-Jiangsu region of eastern China are composed of an alkali basaltic suite consisting of alkali basalts, basanites and nephelinites. The basalts underwent a limited degree of fractional crystallization dominated in the initial stage by olivine and subsequently by clinopyroxene. The magmas were derived from a metasomatized amphibole-bearing garnet peridotite source variously enriched in incompatible trace elements. The metasomatic enrichment of the incompatible trace elements was probably produced by a zone refining process and was superimposed on a depleted upper mantle source compositionally similar to that of mid-ocean ridge basalts.

Geochemistry and origin of Pliocene alkali basaltic lavas from Anhui-Jiangsu, eastern China

Cenozoic alkali basaltic rocks from the central and eastern parts of Shandong province, eastern China include alkali basalts, basanites and olivine nephelinites, some of which contain spinel peridotite xenoliths and megacrysts predominantly of clinopyroxene and anorthoclase. The abundances and ratios of incompatible elements in the basalts are similar to those of many continental and oceanic island alkali basalts. The magmas were derived from metasomatised garnet-clinopyroxene-bearing peridotites by variable but small degrees of melting. A metasomatism which produced an enrichment of strongly incompatible elements was superimposed on a depleted source shortly prior to a melting event. The basaltic rocks from the central and eastern regions have distinct major and trace element characteristics and Sr-Nd-Pb isotopic compositions. The Eastern Shandong lavas have higher abundances of Na, P and incompatible trace elements but are lower in Mg and compatible trace elements than those from the central part. The source of the Eastern Shandong basalts was relatively homogeneous in Sr and Pb isotopic compositions and was metasomatically enriched, at least in part, by carbonate-rich fluids. Phlogopite and probably apatite remained in the residue after the extraction of Eastern Shandong nephelinites. The basaltic rocks of Central Shandong were generated from a heterogeneous multicomponent source. δ18O values negate a significant role of upper crustal contamination in the genesis of the Shandong basalts. The compositional similarities between the Shandong continental alkali basaltic lavas and continental and oceanic island alkali basalts suggest that their mantle parents were metasomatically enriched by fluids derived from the same source—rising asthenospheric plumes.

Geochemistry of Cenozoic alkali basaltic lavas from Shandong Province, eastern China.

The Pliocene volcanic rocks of the Anhui-Jiangsu region of eastern China are composed of an alkali basaltic suite consisting of alkali basalts, basanites and nephelinites. The basalts underwent a limited degree of fractional crystallization dominated in the initial stage by olivine and subsequently by clinopyroxene. The magmas were derived from a metasomatized amphibole-bearing garnet peridotite source variously enriched in incompatible trace elements. The metasomatic enrichment of the incompati ble trace elements was probably produced by a zone refining process and was superimposed on a depleted upper mantle source compositionally similar to that of mid-ocean ridge basalts.

Xiachen Zhi

Papers

Geochemistry and origin of Pliocene alkali basaltic lavas from Anhui-Jiangsu, eastern China

Geochemistry of Cenozoic alkali basaltic lavas from Shandong Province, eastern China.

Geochemistry and origin of Pliocene alkali basaltic from Anhui-Jiangsu, eastern China lavas