Wenli Ling

Recycling lower continental crust in the North China craton

Foundering of mafic lower continental crust into underlying convecting mantle has been proposed as one means to explain the unusually evolved chemical composition of Earths continental crust, yet direct evidence of this process has been scarce. Here we report that Late Jurassic high-magnesium andesites, dacites and adakites (siliceous lavas with high strontium and low heavy-rare-earth element and yttrium contents) from the North China craton have chemical and petrographic features consistent with their origin as partial melts of eclogite that subsequently interacted with mantle peridotite. Similar features observed in adakites and some Archaean sodium-rich granitoids of the tonalite-trondhjemite-granodiorite series have been interpreted to result from interaction of slab melts with the mantle wedge. Unlike their arc-related counterparts, however, the Chinese magmas carry inherited Archaean zircons and have neodymium and strontium isotopic compositions overlapping those of eclogite xenoliths derived from the lower crust of the North China craton. Such features cannot be produced by crustal assimilation of slab melts, given the high Mg#, nickel and chromium contents of the lavas. We infer that the Chinese lavas derive from ancient mafic lower crust that foundered into the convecting mantle and subsequently melted and interacted with peridotite. We suggest that lower crustal foundering occurred within the North China craton during the Late Jurassic, and thus provides constraints on the timing of lithosphere removal beneath the North China craton.

PRECAMBRIAN CRUSTAL GROWTH OF YANGTZE CRATON AS REVEALED BY DETRITAL ZIRCON STUDIES

Clastic sedimentary rocks are natural samples of the exposed continental crust over large areas. The age and evolution of the Yangtze craton is not well known, because much of the craton is covered by thick Nanhua (850 –635Ma), Sinian (635 –543Ma) and Phanerozoic sedimentary rocks. The Nanhua clastic sedimentary rocks including tillites provide ideal samples for studies of age and evolution of the craton. 1130 zircons in 8 sandstone and tillite samples of three Nanhua formations (Liantuo, Gucheng and Nantuo) from the Yangtze Gorges area were dated by LA-ICP-MS. 251 of the 711 concordant zircons were analyzed for Hf isotopic compositions by LA-MC-ICP-MS. The results reveal four major age groups of 720 to 910 Ma, 1.90 to 2.05 Ga, 2.40 to 2.55Ga and 2.60 to 2.70 Ga with few grains >3.2Ga. Although Hf isotopic compositions show both juvenile crustal growth and reworking of old crust for all the age groups, the Paleoproterozoic is a period of prominent crustal reworking with negative εHf(t) values. The Neoproterozoic is a period of significant juvenile crustal additions, accounting for 68 percent of zircons with positive εHf(t) values similar to those of the depleted mantle. Crustal additions at 3.2 to 3.8Ga are also significant, as indicated by the zircon Hf continental model ages. Our results highlight the importance of analysis of a sufficient number of zircons for provenance studies. In addition, our results illustrate that possible temporal and spatial provenance variations have to be taken into account for better characterizing formation and evolution of the related continental crust. Our obtained youngest age for each formation shows a strong negative correlation with stratigraphic height. Youngest ages provide good constraints on the maximum ages of the three Nanhua formations: ≤770Ma, ≤733Ma and ≤704Ma for the base, middle unit and top of the Liantuo Formation, respectively; ≤703Ma for the Gucheng Formation and possibly ≤600 to 606Ma for the Nantuo Formation. Although the North China and Yangtze cratons show some apparently similar zircon ages, the two cratons have distinct histories of formation and evolution. While the North China craton is dominated by Archean crustal growth, the Yangtze craton is characterized by prominent crustal additions in the Neoproterozoic, which is almost lacking in the North China craton.

Geochronology and geochemistry of Grenville-aged (1063 ± 16 Ma) metabasalts in the Shennongjia district, Yangtze block: implications for tectonic evolution of the South China craton

To better understand the evolution of the South China Craton (SCC), we have determined the geochronological and geochemical compositions of newly recognized Grenville-aged metabasalts in the Shennongjia region of the northern Yangtze block. LA-ICP-MS U-Pb dating of zircons indicates that the metabasalts formed at 1063 ± 16 Ma. The rocks are calc-alkaline, are characterized by SiO2 contents (50.50 to 55.62 wt.%), and have moderate-to-high MgO contents (7.25–9.60 wt.%). They display light rare earth element enrichment ((La/Yb)N = 7.0–9.8) with slightly negative Eu anomalies (Eu/Eu* = 0.82–0.90) and have pronounced depletion in high-field strength elements as well as positive Pb anomalies in the primitive mantle-normalized trace element pattern. They possess high initial Sr isotopic ratios of 0.7092–0.7107, large negative εNd(t) values of −12.1 to −11.0, and a relatively narrow range of initial Pb isotope ratios (206Pb/204Pb = 16.503–17.019, 207Pb/204Pb = 15.259–15.452, and 208Pb/204Pb = 36.169–36.994). These isotopic characteristics are typical of basalts derived from an EM2 source region and suggest a subcontinental lithospheric mantle source that was metasomatized by subducted components (fluids and melts). Integrating our new data with documented igneous and metamorphic events during late Mesoproterozoic to early Neoproterozoic time in the region and western segments of the Yangtze block, we suggest that the Shennongjia area might be a microcontinent that was independent of the continental nucleus of the Yangtze block and was accreted to the Yangtze block at the end of Mesoproterozoic time. This study thus argues against the traditional view that the SCC was formed simply by Yangtze–Cathaysia collision and supports a hypothesis in which the Yangtze block was a collage of microcontinents accreted during the Grenvillian period accompanying the assembly of the Rodinia supercontinent.

Decoupled Ce-Nd isotopic systematics of the Neoproterozoic Huangling intrusive complex and its geological significance, eastern Three Gorges, South China

Cerium is one of multivalent rear earth elements, which can transfer from trivalence to tretavalence at oxidizing environment. This process may cause variable degrees of fractionation of Ce from other trivalent rear earth elements, and thus may provide specific insight into the geological processes associated with marked redoxomorphism. Multiple geochemical tracing of Sr-Nd-Ce isotopes are performed on the felsic and mafic intrusives of the Neoproterozoic (~800 Ma) Huangling complex located at the eastern Three Gorges, South China. The intrusive rocks exclusively show various extents of negative Ce anomalies. On the εCe-εNd plot, most samples from the mafic intrusions scatter within the second quadrant, whereas those from the felsic intrusions within the fourth Quadrant. Both of the two groups exhibit relatively large range of εCe(t) variation but limited εNd(t) range, which cause a deviation from the “crustal array” and reveal a decoupled Nd-Ce isotope correlation. The intermediate-felsic suite have varied Ce/Ce* ratios but broadly proximate εCe(t) values, indicating that their negative Ce anomalies were generated during the magmatism; on the contrary, a positive correlation between εCe(t) and Ce/Ce* is observed for the intermediate-mafic suite, an indication of an origin of post-magmatic alteration or metamorphism for their Ce anomalies. Calculation of model age, the occurrence age of negative Ce anomalies (TCe) for the intermediate-mafic samples infers that the alteration events took place >350 Ma. Data showed that negative Ce anomalies of the felsic intrusions may reflect an increase of oxygen fugacity during magma ascending, rather than an inheritance from their source rocks. This explanation implies that the Neoproterozoic magmatism occurred at the continental nucleus of the Yangtze block were developing at a geodynamic context of rapidly regional uplifting.