Neoarchean subduction: A case study of arc volcanic rocks in Qinglong-Zhuzhangzi area of the Eastern Hebei Province, North China Craton

Abstract

We report on volcanic rocks of the Qinglong-Zhuzhangzi area in northeastern Eastern Hebei Province, North China Craton. The supracrustal rock associations in the study area consist of amphibole/biotite plagioclase gneiss, mica (plagioclase) gneiss/schist, biotite K-feldspar gneiss, (pyroxene) plagioclase amphibolite and amphibolite. Twenty-five new mafic, intermediate to felsic samples can be geochemically classified into four groups. Group 1 is similar to fore arc low Ti tholeiite and is characterized by low SiO2 (49.7wt%) and TiO2 (0.43wt%), slightly depleted LREE and MREE pattern with (La/Yb)N ratio of 0.74, (Gd/Yb)N ratio of 0.82. It most likely derived from depleted mantle in a subduction initiation environment. Group 2 is akin to primitive arc tholeiites with low SiO2 (49.7–51.8wt%), slight enrichments of LREE ((La/Sm)N: 1.06–1.53) and negative Nb anomalies ((Nb/La)PM: 0.50–0.76). Group 3 resembles evolved arc tholeiites and displays pronounced enrichments of LREE ((La/Yb)N of 2.38–11.3, (La/Sm)N of 1.70–2.86) and negative Nb, Ta anomalies ((Nb/La)PM: 0.14–0.34, except sample 11JD28-4 with that ratio of 1.00). Group 2 and 3 formed by partial melting of sub-arc depleted mantle wedge modified by differing quantities of slab flux addition. Group 4 comprises K-rich andesite-dacites, which are distinguished by the highest SiO2 (54.0–71.8wt%), K contents (up to 5.03wt%), strong enrichments of LREE ((La/Yb)N ratios of 4.83–19) and negative Nb–Ta anomalies, analogous to shoshonitic rocks marking late stage of subduction.LA-ICPMS zircon U–Pb and Lu–Hf analyses of evolved arc basalts and K-rich andesite-dacites display crystallization ages of 2511–2604Ma and single stage Hf model ages of 2534–2927Ma (most focus on 2650–2800Ma). In contrast to ∼2510–2560Ma arc associations, ∼2604Ma shoshonitic dacites most plausibly represent an older orogenic event. Collectively, supracrustal rocks in the Qinglong-Zhuzhangzi area constitute a series of volcanic arc associations that correspond to an entire subduction cycle ranging from orogen to primitive arc–evolved arc and back to an orogenic setting, which further demonstrate that, by the late Archean, plate tectonic and subduction processes already closely resembled modern counterparts.