Middle-crust accumulation of hornblendite and pyroxenes from slab- and mantle wedge-derived magmas and recharge of high-18O felsic magma collectively generated the Hadanxun arc magmas, northern East Junggar

Abstract

Secondary-ion mass spectrometric U-Pb age and δ18O and laser-ablation multi-collector ICP-MS εHf (t) of zircons, mineral chemistry and whole-rock geochemistry and Nd-Sr isotopes of the Hadanxun intrusive complexes and Bieli'atun pluton, northern East Junggar were analyzed in this study. The Hadanxun intrusive complexes were intruded during Carboniferous (324.8–323.1 Ma) and Early Permian (290.0–281.3 Ma). The Carboniferous intrusive complex varies from diorite (with entrained block of hornblendite cumulate), monzodiorite through quartz monzonite/monzonite to monzogranite. The Early-Permian intrusive rocks include gabbro/dioritic rock, quartz monzonite/monzonite with anorthosite, entrained block of feldspathic pyroxenite and alkali-feldspar granite. The Carboniferous intrusive rocks exhibit decreasing MgO, TFe2O3, TiO2, CaO, Sr and V with increasing SiO2. Their rare earth element (REE) distribution patterns are concave-up and steeply-dipping with decreasing Dy/Yb and primitive-mantle normalized NbN/TaN. These characteristics can be modeled with two stage fractional crystallization early of hornblendite and late of feldspar-dominated minerals. These rocks all plot within adakite field in the Sr/Y vs. Y diagram, suggesting derivation from subducting oceanic-slab.The Early-Permian intrusive rocks with pronounced Nb, Ta and Ti negative anomalies and depleted isotopic compositions manifest arc affinity with derivation from slab fluid-metasomatized mantle wedge. The feldspathic pyroxenites exhibit negative Eu anomaly (Eu/Eu* = 0.57–0.78), flat HREE distribution pattern with relative enrichment of HREE, significant negative Sr, P anomalies and REE = 157.8–186.7 ppm and Zr = 97.9–177. Just the opposite, the anorthosite and quartz monzonite/monzonite manifest remarkable positive Eu anomalies (Eu/Eu* = 1.27–1.47), steep REE distribution pattern with depleted HREE, positive Sr, P anomalies and REE = 167–189.8 and Zr = 92.9–141. The complementarity of the two categories of rock with high REE and Zr suggests that they have equilibrated via a crustal melt-buffered liquid. In the quartz monzonite/monzonite, extensive K-, Na-metasomatism of plagioclase and metasomatism of hornblende into biotite suggest mixing of K-, Na- and Si-rich felsic magma into Al-, Ca-rich magma. Quantitative modeling can reproduce the complementarity of the two categories of rock with alkali-feldspar granite as Stage 2 residual melt. Underplating coupled with interaction of high-18O fluid caused hydrogen metasomatism and partial melting of juvenile lower-crust generating K-, Na- and Si-rich felsic magma. The latter was replenished to magma plumbing system and drove the preceding magma to ascend together. Meanwhile, the felsic magma mixed with pyroxene crystal mush in the upper stream and Al-, Ca-rich derivative magma in the down stream. This eventually formed lower cumulate beneath upper cumulate at emplacement, i.e. feldspathic pyroxenite (cumulus pyroxenes cemented by mesoperthite) beneath quartz monzonite/monzonite plus anorthosite of the Hadanxun main pluton. Al-in-hornblende barometry and two-pyroxene thermobarometry reveal that hornblendite (population 1 hornblendes: 26–22 km depth; population 2: 20–18 km) and pyroxenite (23–16 km) cumulates constitute high P-wave velocity body (27 to 15 km depth) in the middle crust of northern East Junggar, whereas fragment of hornblendite and symplectitic pyroxenes (18–8 km) record decompression when they were brought through magma plumbing system. The Hadanxun intrusive rocks including mixed product i.e. quartz monzonite/monzonite exhibit clustered εNd (t) (6.2–6.6) and (87Sr/86Sr)i (0.7037–0.7040) that are much more depleted than the Armantai ophiolite and Chinese-Altay continental-crustal materials. This precludes Early-Paleozoic oceanic crust and continental-crustal materials from Chinese Altay as source rock. The K-, Na- and Si-rich felsic melt was derived from newly-underplated material.Three post-Carboniferous intrusions (weighted mean zircon δ18O = 6.96‰–7.65‰) that are controlled by Kalaxiangeer fault system (F3, F4) record significant δ18O increase over Carboniferous intrusions (3.64‰–6.42‰) that are away from F3 and F4. However, both of them exhibit rather depleted Nd-Sr-Hf isotopic compositions. This does not reveal significant compositional change in the lower-crustal source rock during Early Permian that would have occurred if the northern East Junggar arc terrain had collided with the Chinese-Altay continental-margin. Rather, the latter two were joined by strike-slip such that high-18O fluid from the Chinese Altay could migrate to the northeastern East Junggar. Incorporation of the fluid or fluid-interacted crustal material increased δ18O of the post-Carboniferous intrusions.