Southwestern Yangtze Block Research Articles

“Grenvillian” intra-plate mafic magmatism in the southwestern Yangtze Block, SW China

Tectonic affinity of Grenvillian magmatism in the South China Block is important for understanding the role of the block in the reconstruction of the Rodinia supercontinent during the Mesoproterozoic. In the southwestern Yangtze Block, South China, late Mesoproterozoic metabasalts occur as layers in the Pudeng Formation of the Julin Group. Most metabasalts are highly metamorphosed to be amphibolites composed of dominantly plagioclase and amphibole with minor Fe–Ti oxides, titanite and apatite. Two metabasalt samples have identical zircon U–Pb ages of ∼1050Ma. These basalts are tholeiitic in compositions and have high and variable TiO2 (1.1–3.2wt.%) and low Mg# values (=Mg/(Mg+Fe2+); 35–60). In the primitive mantle-normalized diagram, they are characterized by enrichments in Th, Nb, Ta and light REE without significant Zr–Hf–Nb–Ta–Ti anomalies. The samples have ɛNd(t) values ranging from −1.4 to +4.0 and their zircon grains have ɛHf(t) values from −1.7 to +11.0. The low Mg# values and Cr (55.1–261ppm) and Ni (13.5–76.5ppm) contents suggest fractional crystallization of olivine and/or clinopyroxene, whereas minor negative Eu anomalies indicates slightly fractional crystallization of plagioclase. High positive ɛNd(t) and ɛHf(t) values suggest derivation of the parental magma from a depleted asthenospheric mantle source. On the other hand, variable ɛNd(t) and ɛHf(t) values and La/Sm and Nb/La ratios are consistent with significant crustal assimilation by the Paleoproterozoic rocks in the region. These samples have high Ti and Ti/V ratios (mostly >40), typical of within-plate basalts. In the Zr–Nb–Y and Th–Ta–Hf diagrams for tectonic discrimination, the samples with least crustal contamination are plotted in the ‘within-plate’ and ‘E-MORB’ fields, indicating an intra-continental rifting setting. In view of subsequent Neoproterozoic arc magmatic activities (950–730Ma) in the region, we suggest that these basalts were likely produced at a rifting basin in a passive margin. Results presented here demonstrate that the Grenville-age magmatism in the southwestern Yangtze Block was not orogenic in origin, thus arguing against the existence of Grenvillian Orogen in the region that was traditionally used to place the South China Block in the central part of the Rodinia supercontinent.

Provenance and tectonic setting of the Paleo- to Mesoproterozoic Dongchuan Group in the southwestern Yangtze Block, South China: Implication for the breakup of the supercontinent Columbia

Abstract The Paleoproterozoic to Mesoproterozoic (1742–1503 Ma) Dongchuan Group in the southwestern Yangtze Block is a rift-related sedimentary sequence that was associated with the breakup of the supercontinent Columbia and is particularly important for the possible linkage between the Yangtze Block and other continents in Columbia. The Dongchuan Group consists of the Yinmin, Luoxue, Etouchang and Luzhijiang formations from the base upward. Sandstones from the Yinmin Formation are mainly arkose containing dominant K-feldspar with subordinate plagioclase and quartz. Abundant feldspar and high Qm/Q ratios (0.94–1) are indicative of plutonic sources. These sandstones have high La/Sc (3.06 to 4.32), low Sc/Th (0.74 to 1.15) and Co/Th (0.85 to 1.52) and highly evolved Nd isotopes (eNd(t) = − 6.2 to − 8.2), consistent with an old, felsic igneous source. Detrital zircons of this formation have two major age groups at 2602–2887 Ma and 2224–2392 Ma. Siltstones of the Etouchang Formation have detrital zircons with a prominent age peak at ~ 2560 Ma and several subordinate peaks at ~ 2180 Ma, ~ 2100 Ma and ~ 1900 Ma. They have high Sc/Th (1.00–7.08), Co/Th (0.13 to 6.31) and eNd(t) (− 2.1 to − 6.7), significantly different from the Yinmin Formation. The Yinmin Formation is interpreted to deposit during the initial stage of extensional rifting receiving detritus of granites and TTG mainly from uplifted shoulder. The Etouchang Formation more likely formed in a passive margin with sedimentary material largely from cratonic sources. Paleoproterozoic to Mesoproterozoic rift basins in the southwestern Yangtze Block, north Australia and northwestern Laurentia have remarkably similar provenance and tectonic setting in their lower part (1742–1596 Ma), but significantly different since the onset of the Etouchang Formation (ca. 1596 Ma). Therefore, the southwestern Yangtze Block was likely connected with the north Australia and northwestern Laurentia in Columbia and drifted away from these continents on or before ca. 1596 Ma.

Revisiting the “Yanbian Terrane”: Implications for Neoproterozoic tectonic evolution of the western Yangtze Block, South China

There is a consensus that the nature and evolution of the widespread Neoproterozoic igneous rocks along the western margin of the Yangtze Block are important for understanding the tectonic evolution of South China, but the petrogenesis and tectonic affiliations of these rocks are still highly debated. The Yanbian region in the southwestern Yangtze Block preserves Neoproterozoic igneous packages that provide a rare opportunity for unraveling the regional tectonomagmatic evolution. SHRIMP U–Pb zircon data show two major episodes of mafic–ultramafic magmatism in the region. The elder one is represented by a basaltic suite within the Yanbian Group that formed at ca. 920–900(?) Ma. Geochemical and Nd isotopic data suggest these basalts were derived from a highly depleted asthenospheric mantle in a continental back-arc basin, rather than being part of an ophiolite suite formed in an oceanic setting as previously suggested. A major fold and thrust event took place during the final stage of the Sibao orogenesis at ca. 900 Ma, forming the east-northeasterly trending structures within the Yanbian Group rocks (and regional pre-900 Ma rocks as well). The younger magmatic episode is represented by the non-deformed mafic–ultramafic intrusions dated at 820–800 Ma. Mineral compositions and whole-rock geochemistry suggest that they are affinitive with rift-related, rather than arc-related, magmatism, with some pre-existing arc components being assimilated during their genesis. These mafic–ultramafic intrusions belong to the ca. 830–740 Ma anorogenic igneous provinces along the north-south trending Kangdian Rift in the western Yangtze Block. The igneous rocks in the Yanbian region had thus recorded an important tectonic transition from a back-arc setting during the Sibao Orogeny at ≥900 Ma to a continental rift setting after ca. 825 Ma in the southwestern Yangtze Block. This tectonic transition most likely corresponded to the amalgamation and breakup of the Rodinia supercontinent. We see neither geological nor geochemical evidence for the existence of a mid-Neoproterozoic (830–740 Ma) magmatic arc or an ophiolitic complex in the Yanbian region as suggested by previous studies.

Crustal structure of Gondwana- and Yangtze-typed blocks: An example by wide-angle seismic profile from Menglian to Malong in western Yunnan

The wide-angle seismic profile between Menglian and Malong crosses the Baoshan block (Gondwana-typed), and Simao and southwestern Yangtze blocks (Yangtze-typed). By interpreting the wide-angle seismic data, we obtained the seismic crust/upper mantle structure of P-wave velocities together with the seismic reflections of these three blocks, Changning-Menglian and Mojiang suture zones among the mentioned three blocks. Our interpreting results demonstrate that the P-wave crustal velocity of Simao block is slower than that of Baoshan and southwestern Yangtze block and the crustal thickness gradually thickens from the Baoshan block, Simao to southwestern Yangtze block. Crustal reflection patterns of these three blocks have distinct differences too. For the Gondwana-typed blocks, seismic reflections in the upper crust are well developed while in middle-lower crust they are very weak. The crustal reflections in the Yangtze block are very well developed. The crustal reflection patterns in Simao and southwestern Yangtze blocks are distinguishable. The average thickness of the crust in the studied area is about 40 km. And we make some discussions on the crustal thickening model of the three blocks in western Yunnan and tectonic setting of seismic developing and interaction of Gondwana and Yangtze blocks