Major, Volatile, Ore, and Trace Elements in Magmatic Melts in the Earth’s Dominant Geodynamic Environments. I. Mean Concentrations

Here, we investigate early Permian adakites from Inner Mongolia, North China. The adakites are found along the Hegenshan suture zone, were emplaced into the late Carboniferous Meilaotewula suprasubduction zone (SSZ) ophiolite and consist mainly of fine‐ to medium‐grained granodiorites. U–Pb zircon dating reveals that the Meilaotewula adakites crystallized at 292 Ma. The adakites belong to the low‐K tholeiitic and medium‐K calc‐alkaline series. They are characterized by high SiO 2 (65.20–70.05 wt.%), Al 2 O 3 (16.46–19.27 wt.%), and Sr (343–666 ppm); low MgO (1.26–1.95 wt.%), Yb (0.65–1.16 ppm), and Y (8.08–10.6 ppm) contents; and Na 2 O/K 2 O ratios (3.26–10.73). They are relatively enriched in large‐ion lithophile elements, such as K, Rb, and Sr, and depleted in high‐field‐strength elements, such as Nb, Ta, Zr, Ti, and P, and have low total rare‐earth element (REE) contents (27.73–49.63 ppm), with distinct REE fractionation (chondrite‐normalized La/Yb of 3.05–8.88) and no pronounced negative Eu anomalies. The rocks have relatively low initial 87 Sr/ 86 Sr values (0.70285–0.70326), high ε Nd ( t ) values (+8.8 to +10.8), and relatively high zircon ε Hf ( t ) values (11.9 to 15.9), indicating that the magma was derived from young juvenile oceanic crust derived from depleted mantle, similar to adakitic rocks formed by partial melting of subducted oceanic crust. The relatively high Mg # of the adakites indicates that the melts interacted with mantle peridotite during ascent. The adakites, together with Meilaotewula ophiolite (308 Ma), may have formed during the early stages of intra‐oceanic subduction, demonstrating that subduction initiation in the southeastern Palaeo‐Asian Ocean occurred during the late Carboniferous to early Permian.

Adakite

Primitive mantle

10.1002/gj.3696

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Citations (6)

Petrogenesis of Adakitic Porphyries in an Extensional Tectonic Setting, Dexing, South China: Implications for the Genesis of Porphyry Copper Mineralization

Journal of Petrology (2005)

Qiang Wang Ji‐Feng Xu Ping Jian Zhiwei Bao Z. Zhao

The Dexing adakitic porphyries (quartz diorite–granodiorite porphyries), associated with giant porphyry Cu deposits, are located in the interior of a continent (South China). They exhibit relatively high MgO, Cr, Ni and Sr contents, high La/Yb and Sr/Y ratios, but low Yb and Y contents, similar to adakites produced by slab melting associated with subduction. However, they are characterized by bulk Earth-like Nd–Sr isotope compositions (εNd(t) = −1·14 to +1·80 and (87Sr/86Sr)i = 0·7044 – 0·7047), and high Th (12·6–27·2 ppm) contents and Th/Ce (0·19–0·94) ratios, which are different from those of Cenozoic slab-derived adakites. Sensitive High-Resolution Ion Microprobe (SHRIMP) geochronology studies of zircons reveal that the Dexing adakitic porphyries have a crystallization age of 171 ± 3 Ma. This age is contemporaneous with Middle Jurassic extension within the Shi-Han rift zone, and within-plate magmatism elsewhere in South China, indicating that the Dexing adakitic porphyries were probably formed in an extensional tectonic regime in the interior of the continent rather than in an arc setting. Their high Th contents and Th/Ce ratios, and Middle Jurassic age, argue against an origin from a Neoproterozoic (∼1000 Ma) stalled slab in the mantle. Taking into account available data for the regional metamorphic–magmatic rocks, and the present-day crustal thickness (∼31 km) in the area, we suggest that the Dexing adakitic porphyries were most probably generated by partial melting of delaminated lower crust, which was possibly triggered by upwelling of the asthenospheric mantle due to the activity of the Shi-Hang rift zone. Moreover, the Dexing adakitic magmas must have interacted with the surrounding mantle peridotite during their ascent, which elevated not only their MgO, Cr and Ni contents, but also the oxygen fugacity (fO2) of the mantle. The high fO2 could have induced oxidation of metallic sulfides in the mantle and mobilization of chalcophile elements, which are required to produce associated Cu mineralization. Therefore, the Cu metallogenesis associated with the Dexing adakitic porphyries is probably related to partial melting of delaminated lower crust, similar to the metallogenesis accompanying slab melting.

Adakite

Petrogenesis

Diorite

Geochronology

Primitive mantle

Rodinia

10.1093/petrology/egi070

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Citations (801)

Adakites, High-Nb Basalts and Copper–Gold Deposits in Magmatic Arcs and Collisional Orogens: An Overview

Geosciences (2022)

Pavel Kepezhinskas Н. В. Бердников Nikita Kepezhinskas N. S. Konovalova

Adakites are Y- and Yb-depleted, SiO2- and Sr-enriched rocks with elevated Sr/Y and La/Yb ratios originally thought to represent partial melts of subducted metabasalt, based on their association with the subduction of young (<25 Ma) and hot oceanic crust. Later, adakites were found in arc segments associated with oblique, slow and flat subduction, arc–transform intersections, collision zones and post-collisional extensional environments. New models of adakite petrogenesis include the melting of thickened and delaminated mafic lower crust, basalt underplating of the continental crust and high-pressure fractionation (amphibole ± garnet) of mantle-derived, hydrous mafic melts. In some cases, adakites are associated with Nb-enriched (10 ppm < Nb < 20 ppm) and high-Nb (Nb > 20 ppm) arc basalts in ancient and modern subduction zones (HNBs). Two types of HNBs are recognized on the basis of their geochemistry. Type I HNBs (Kamchatka, Honduras) share N-MORB-like isotopic and OIB-like trace element characteristics and most probably originate from adakite-contaminated mantle sources. Type II HNBs (Sulu arc, Jamaica) display high-field strength element enrichments in respect to island-arc basalts coupled with enriched, OIB-like isotopic signatures, suggesting derivation from asthenospheric mantle sources in arcs. Adakites and, to a lesser extent, HNBs are associated with Cu–Au porphyry and epithermal deposits in Cenozoic magmatic arcs (Kamchatka, Phlippines, Indonesia, Andean margin) and Paleozoic-Mesozoic (Central Asian and Tethyan) collisional orogens. This association is believed to be not just temporal and structural but also genetic due to the hydrous (common presence of amphibole and biotite), highly oxidized (>ΔFMQ > +2) and S-rich (anhydrite in modern Pinatubo and El Chichon adakite eruptions) nature of adakite magmas. Cretaceous adakites from the Stanovoy Suture Zone in Far East Russia contain Cu–Ag–Au and Cu–Zn–Mo–Ag alloys, native Au and Pt, cupriferous Ag in association witn barite and Ag-chloride. Stanovoy adakites also have systematically higher Au contents in comparison with volcanic arc magmas, suggesting that ore-forming hydrothermal fluids responsible for Cu–Au(Mo–Ag) porphyry and epithermal mineralization in upper crustal environments could have been exsolved from metal-saturated, H2O–S–Cl-rich adakite magmas. The interaction between depleted mantle peridotites and metal-rich adakites appears to be capable of producing (under a certain set of conditions) fertile sources for HNB melts connected with some epithermal Au (Porgera) and porphyry Cu–Au–Mo (Tibet, Iran) mineralized systems in modern and ancient subduction zones.

Adakite

Underplating

Petrogenesis

Primitive mantle

Island arc

10.3390/geosciences12010029

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Citations (28)

Slab melting in the Aleutians: implications of an ion probe study of clinopyroxene in primitive adakite and basalt

Earth and Planetary Science Letters (1998)

Gene M. Yogodzinski P. B. Kelemen

Adakite