The late Archean (~3.0–2.5 Ga) was a key period of continental growth globally, which is widely considered to reflect the onset of vigorous plate tectonic activity, although related continental growth modes remain contentious. Here we investigate a suite of late Neoarchean metavolcanic rocks from the southwest Qixia area of the Jiaobei terrane in the North China Craton. The rocks in this suite include amphibolites, clinopyroxene amphibolites, and hornblende plagioclase gneisses. We present zircon U-Pb isotopic data which indicate that the protoliths of these rocks formed during ~2549–2511 Ma.The (clinopyroxene) amphibolites correspond to meta-basaltic rocks, with some containing high modal content of titanite. These rocks show moderate to high FeOT (8.96–13.62 wt.%) and TiO2 (0.59–1.59 wt.%), flat to less fractionated REE patterns, and mildly negative Th, Nb, and Ta anomalies, resembling those of Fe-tholeiites. In addition, they display positive zircon ?Hf(t) values (+2.6 to +8.7), and are devoid of crustal contamination or fractional crystallization. Combined with the low Nb/Yb (mostly < 1.60) and (Hf/Sm)N (mostly < 0.95), low to moderate Th/Yb (0.08–0.54), and low V/Sc (5.53–9.19) ratios, these basaltic rocks are interpreted to have been derived from a relatively reduced and depleted mantle source that was mildly metasomatized by hydrous fluids. The hornblende plagioclase gneisses are meta-andesitic rocks, and occur interlayered with the basaltic rocks. They are transitional between tholeiitic and calc-alkaline rock series, and show fractionated REE patterns with evidently negative Th, Nb, and Ta anomalies. The depleted zircon ?Hf(t) values (+2.4 to +8.4) and quantitative chemical modeling suggest that the andesitic rocks were most likely generated by injection and mixing of juvenile felsic magmas with the tholeiitic basaltic magmas.In general, the chemical features and genesis of late Neoarchean meta-basaltic rocks in our study area resemble those of Mariana back-arc basin basalts. Combined with regional geological data, it is proposed that the Jiaobei terrane witnessed late Neoarchean crustal growth under a paired continental arc-back arc setting. On a regional context, we propose two distinct geodynamic mode of late Neoarchean continental growth across North China Craton (particularly the Eastern Block), i.e., (1) arc-continent accretion along northwestern part of the Eastern Block; and (2) paired continental arc-back arc system surrounding the ~3.8–2.7 Ga continental nuclei to the southeast. 相似文献
Acta Geotechnica - This paper is concentrated on the influence of the inner soil on the torsional vibration of a pipe pile embedded in inhomogeneous soil. First, the soil–pile system is... 相似文献
In certain field conditions such as offshore projects under wave loads or embankments under traffic loads, both the vertical and horizontal stresses are variable. However, previous investigations rarely considered the variation in horizontal stress. To better understand the characteristics of natural saturated soft clay, a series of monotonic and cyclic triaxial tests with a K0-consolidation state were carried out under a variable confining pressure (VCP) stress path. The development of axial strain, pore water pressure and effective stress path is analysed. The results show that with the increase in η (the ratio of the variation in the mean effective principal stress to that of the deviatoric stress), the undrained shear strength (qf) decreases continuously. The pore water pressure generation is slightly improved under a stress path with increasing confining pressure. Based on the test results, a unified formula was established to predict the pore water pressure under VCP stress paths. The unique p–q–e relationship of normally consolidated clay in monotonic VCP triaxial tests was also demonstrated. Under VCP stress paths, the amplitude of the pore pressure increases, and the effective stress path tilts more sharply to the right. Moreover, a unified formula was established that can provide a good reference for predicting effective stress paths under cyclic VCP triaxial tests.
