Molybdenum and Boron Isotopic Compositions of Porphyry Cu Mineralization‐Related Adakitic Rocks in Central‐Eastern China: New Insights Into their Petrogenesis and Crust‐Mantle Interaction

Abstract

The voluminous Late Mesozoic porphyry ore-related adakitic magmatism in central-eastern China is crucial to understand the deep geodynamic process of the region and controls on porphyry deposit formation globally. Here, we present a novel study of whole-rock MoB isotopes on the mineralization-related adakitic rocks from the Dexing (located in the interior of South China and associated with giant porphyry Cu deposits), and Middle-Lower Yangtze River (MLYR) (host to one of the largest porphyry Cu-Au deposit belts) areas in central-eastern China. The Dexing adakitic rocks have δ 98/95 Mo ranging from −0.31‰ to −0.01‰ and δ 11 B from −17.65‰ to −11.49‰. The adakitic rocks from the MLYR area have δ 98/95 Mo values of −0.5‰ to 0.2‰ and δ 11 B values of −14.48‰ to −9.53‰. The δ 98/95 Mo values of both the Dexing and MLYR samples negatively correlate with the La/Yb and La/Sm ratios. Together with the high Mg#, Cr, and Ni contents, these results indicate a process of melt-mantle interaction resulting in hybridization of initial adakitic melts with low δ 98/95 Mo and mantle-derived melts/components with high δ 98/95 Mo. These adakitic rocks have distinct Mo isotope systematics compared with those of oceanic slab-derived adakites and MORB-type eclogites. Their relatively low δ 11 B values and the lack of correlation between δ 11 B and B contents support a mafic lower crust source for the initial adakitic melts. We thus suggest that these adakitic rocks were generated by partial melting of delaminated lower crust followed by interaction with the deep mantle, and that melt-mantle interaction may facilitate Cu-Mo-Au mineralization.

Re-assessment of the effect of fractional crystallization on Mo isotopes: Constraints from I-type granitoids and their enclosed mafic magmatic enclaves

“…A previous study has observed in double spiked isotope analyses, average compositions of the reference standard within a measurement session often deviate slightly from zero (e.g., Hin et al, 2013). This drift was corrected by subtracting the average composition of NIST SRM 3134 in (W-2a, BHVO-2, GSP-2, AGV-2, see Table A2), which were analyzed with our samples during the measurements sessions, are in agreement with previously published values within error (Bezard et al, 2016;Burkhardt et al, 2014;Fan et al, 2020;Gaschnig et al, 2017;König et al, 2016;Liang et al, 2017;Yang et al, 2015;Chen et al, 2019;Kaufmann et al, 2021). The total procedural blanks are 0.2ng to 0.48 ng for Mo, which is negligible for the samples.…”

“…Yang et al (2017) observed that δ 98/95 Mo was negatively correlated with Fe 2 O 3 contents and K/Rb ratios for I-type granites from the Lachlan Fold Belt in Australia and the Loch Doon, Criffell and Fleet pluton in Scotland and suggested that light Mo isotopes are preferentially incorporated in hornblende and Fe 3+ -rich minerals during granitic magma differentiation. In contrast, Fan et al (2020) showed the absence of correlations between δ 98/95 Mo values and J o u r n a l P r e -p r o o f magma differentiation indicators in granodioritic porphyries and granodiorite from centraleastern China and concluded that fractional crystallization cannot account for Mo isotopes variation in these granitic rocks.…”

“…Although equilibrium mass-dependent isotope fractionation is suggested to be smaller at high temperatures, recent studies on high-temperature materials, such as komatiites (Greber et al, 2015;McCoy-West et al, 2019), oceanic basalts (Liang et al, 2017;Bezard et al, 2016), Icelandic lavas (Yang et al, 2015), arc lavas (Voegelin et al, 2014;Freymuth et al, 2015Freymuth et al, , 2016König et al, 2016;Wille et al, 2018;Villalobos-Orchard et al, 2020), molybdenite deposits (Breillat et al, 2016), and granites (Yang et al, 2015;Fan et al, 2020) have shown that δ 98/95 Mo (relative to NIST SRM 3134 standard) variation in magmatic systems exceeds 2‰. The δ 98/95 Mo of arc lavas show considerable variation, with a span greater than 1.5‰ and a weighted mean of 0.07 ± 0.68‰ (N = 133, 2SD;Villalobos-Orchard et al, 2020), significantly higher than that of the depleted mantle (-0.21 ± 0.02‰; Willbold and Elliott, 2017).…”

Tracing black shales in the source of a porphyry Mo deposit using molybdenum isotopes

“…S3&S4. Replicated digestions and measurements of geological reference materials (GRM) (W-2a, BHVO-2, GSP-2, AGV-2 and BCR-2 see Table S3), which were analyzed with our samples during the measurements sessions, are in agreement with previously published values within error (Bezard et al, 2016;Burkhardt et al, 2014;Chen et al, 2019;Fan et al, 2020;Freymuth et al, 2016;Freymuth et al, 2015;Gaschnig, 2017;Kaufmann et al, 2021;König et al, 2016;Li et al, 2021) Figure (Steiger and Jä ger, 1977). b 143 Nd/ 144 Nd(i)=[( 143 Nd/ 144 Nd)-( 143 Sm/ 144 Nd)(e λt -1)], εNd(t)=[( 143 Nd/ 144 Nd)/( 143 Nd/ 144 NdCHUR)-1]×10000, where λ( 147 Sm)=6.54×10 -12 yr -1 (Lugmair and Marti, 1978), 147 Sm/ 144 NdCHUR=0.1967, 143 Nd/ 144 NdCHUR= 0.512638, 147 Sm/ 144 NdDM = 0.2137, 143 Nd/ 144 NdDM = 0.51315.…”

Subducted oceanic slab break-off in a post-collisional setting: Constraints from petrogenesis of Late Carboniferous dykes in central West Junggar, Xinjiang, NW China

“…However, compared with typical adakitic rocks, the Group 1 dykes have low (La/Yb) N ratios, which was most likely caused by partial melting of peridotite mantle metasomatized by oceanic slab-derived adakitic melts (Ma et al 2012; Yin et al 2013). This indicates that the melt-mantle interaction may have diluted their adakitic features (Fan et al 2020). Based on a minimum pressure (1.5GPa) for rutile appearance, the depth for Group 1 sample production via melting of subducted oceanic crust cannot exceed 50 km (Xiong et al 2005; Xiong, 2006).…”