Taiping Zhao

Zircon U-Pb SHRIMP dating for the volcanic rocks of the Xiong’er Group: Constraints on the initial formation age of the cover of the North China Craton

The volcanic rocks of the Xiong’er Group occur widely in the southern part of the North China Craton, which mark the beginning of the cover in the southern part of the North China Craton. The age of the volcanic rocks is thus crucial to understand the tectonic regime and evolutionary history of the North China Craton in the Proterozoic age. Zircons from five volcanic rocks and intrusions were dated by U-Pb SHRIMP method. The results indicate that the Xiong’er Group formed in 1.80-1.75 Ga of Paleo-Proterozoic. Since the Xiong’er Group formed earlier than the Changcheng System, the earliest rocks in the Changcheng System is therefore assumed to be formed in 1.75 Ga. A thermal-tectonic event of ca. 1.84 Ga is indicated by new zircon U-Pb SHRIMP ages in the southern part of the North China Craton. The volcanic rocks of the Xiong’er Group thus represent the initial magmatism of the Paleo-Proterozoic breakup of the North China Craton. Numerous inherited zircons in the volcanic rocks mainly formed in ∼2.20 Ga, indicating that the source magma of the volcanic rocks may be derived from the ∼2.20 Ga crust, or from a mantle magma with significant contamination of the ∼2.20 Ga crust.

Crustal-contaminated komatiitic basalts in Southern China: products of a Proterozoic mantle plume beneath the Yangtze Block

Abstract Abundant mafic and ultramafic rocks including basalts, komatiitic basalts, and peridotites occur in the Proterozoic Sibao Group, northern Guangxi Province, China. Whereas the basalts are generally pillow lavas, the komatiitic basalts are typically spinifex-textured and, in a few cases, show pyroxene accumulation associated with NiCu(PGE) sulfide deposits. The peridotites occur in the lower portions of differentiated sills, which contain gabbro and diorite in their upper parts. The sills are believed to be co-magmatic with the komatiitic basalts. The spinifex rocks of the Jiepai and Hejia Flows have MgO ranging from 8.9 to 14.3 wt%. The Zhongkui Flow is highly fractionated to form a spinifex zone with lower MgO (5.3–5.9 wt%) and a cumulate zone with higher MgO (17.3–17.9 wt%). Overall the rocks have TiO2=0.44–0.74 wt%. Relative to primitive mantle, they are enriched in Th and LREE, but exhibit negative Ti-, Nb-, and P-anomalies. These features are consistent with their formation from a crustally-contaminated komatiitic magma. During this process of crustal contamination, the magmas assimilated sulfur from sediments, which caused sulfide-segregation resulting in the formation of NiCu(PGE) sulfide deposits. The occurrence of the komatiitic basalts in the Sibao Group can be explained by the ascent of a mantle plume beneath a continental rift environment, and implies that the Yangtze Block may have had an Archean basement through which the Sibao komatiitic basalts erupted.

Geochronology and geochemistry of the c. 80 Ma Rutog granitic pluton, northwestern Tibet: implications for the tectonic evolution of the Lhasa Terrane

The Rutog granitic pluton lies in the Gangdese magmatic arc in the westernmost part of the Lhasa Terrane, NW Tibet, and has SHRIMP zircon U-Pb ages of c. 80 Ma. The pluton consists of granodiorite and monzogranite with SiO2 ranging from 62 to 72 wt % and Al2 O3 from 15 to 17 wt %. The rocks contain 2.33-4.93 wt % K2O and 3.42-5.52 wt % Na2O and have Na2O/K2O ratios of 0.74-2.00. Their chondrite-normalized rare earth element (REE) patterns are enriched in LREE ((La/Yb)n = 15 to 26) and do not show significant Eu anomalies (δEu = 0.68-1.15). On a primitive mantle-normalized trace element diagram, the rocks are rich in large ion lithophile elements (LILE) and poor in high field strength elements (HFSE), HREE and Y. Their Sr/Y ratios range from 15 to 78 with an average of 30. The rocks have constant initial 87 Sr/ 86 Sr ratios (0.7045 to 0.7049) and slightly positive eNd(t) values (+0.1 to +2.3), similar to I-type granites generated in an arc setting. The geochemistry of the Rutog pluton is best explained by partial melting of a thickened continental crust, triggered by underplating of basaltic magmas in a mantle wedge. The formation of the Rutog pluton suggests flat subduction of the Neo-Tethyan oceanic lithosphere from the south. Crustal thickening may have occurred in the Late Cretaceous prior to the India-Asia collision.

Isotopically enriched N-MORB: A new geochemical signature of off-axis plume-ridge interaction—A case study at 50°28′E, Southwest Indian Ridge

Interaction between the Southwest Indian Ridge (46°E and 52°20′E) and Crozet hotspot has been proposed by geophysical studies but remains controversial mostly due to the lack of E-MORB (enriched mid-ocean ridge basalts). Forty-seven new samples collected from this region, including 15 from segment 27 centered at 50°28′E with a 10 km thick crust, are all N-MORB (normal MORB) and can be classified into two groups: a high-Al group only at 50°28′E and a Main group widespread. The former, with higher Al2O3 and lower TiO2 and SiO2, have slightly enriched Sr-Nd-Hf-Pb isotopic compositions. We propose that their major and trace elemental signatures were modified by reaction with primitive cumulate in the crust, whereas the enriched isotopic compositions indicate the contribution of Crozet plume materials. During upslope flow of the Crozet plume to the ridge, decompression melting would occur along the path, which would deplete the plume in incompatible elements but not significantly change the isotopic compositions. Thus, when they finally reach the ridge, the depleted residue would remelt due to further decompression at MOR and produce isotopically enriched N-MORB at segment 27. Isotopically enriched N-MORB are known elsewhere, mostly at slower-spreading ridges possibly influenced by plumes with large plume-ridge distances. In particular, the constant Nd isotopic compositions with decreasing (La/Sm)N ratios for off-axis magmatism between the Reunion hotspot toward the CIR perfectly match such a plume-ridge interaction model. Therefore, aside from E-MORB, isotopically enriched N-MORB can also be considered as the geochemical signature for off-axis plume-ridge interaction.

The Paleoproterozoic Continental Evolution in the Southern North China Craton: Constrains from Magmatism and Sedimentation

Recently, increasing lines of evidence for the 2.45–2.00 Ga magmatic rocks and Paleoproterozoic low-grade metasedimentary sequences have been identified, which can provide diagnostic constrains on the debate on the Paleoproterozoic (2.45–2.00 Ga) evolutional regime of the NCC. The widespread 2.45–2.20 Ga magmatisms occurred in the southern NCC mainly include TTG, dioritic-gabbroic gneiss, amphibolite, and high-K granites. The 2.45–2.20 Ga TTG or TTG-like gneisses show variable Mg# values, low Cr, Ni, and high Rb/Sr ratios, suggesting that they most likely derived from partial melting of basaltic lower crust with juvenile materials addition. The 2.45–2.20 Ga dioritic-gabbroic gneisses show the similar geochemical characteristics with adakitic rocks from thickened lower crust. Their e Hf(t) and e Nd(t) values are variable, and have weak Ta enrichment, and not obvious negative Nb anomalies, suggesting they were produced by partial melting of metasomatized lithospheric mantle. The 2.45–2.20 Ga amphibolites are consistent with magma derivation from MORB-like mantle wedge. The 2.45–2.20 Ga (high-K) calc-alkaline granites are representative of syn-collisional granites, and derived from older crust with variable mixing of a juvenile melt in a subduction-collision related setting. The 2.20–2.00 Ga magmatism reveals a major period of crustal reworking, rather than one of crustal addition. The 2.20–2.00 Ga monzonites have mixed IAB- and OIB-like geochemical signatures, possibly related to extension and thinning of the lithosphere and upwelling of asthenosphere. The 2.20–2.00 Ga potassic granites belong to highly fractionated aluminous A-type granite, and formed in an extensional-rift setting. As indicated by the zircon in situ Hf isotopic compositions, the injection of basaltic melt into the crust has been widely considered as an important mechanism to generate silicious melts. The 2.20–2.00 Ga tonalite would be derived from partial melting of delaminated lower crust. The temporal change from mostly 2.45–2.20 Ga low-K igneous rocks (TTG) to 2.20–2.0 Ga mostly high-K igneous rocks in the southern segment of the NCC indicates a tectonic transformation from accretionary orogenesis (ca. 2.30 Ga) to extensional regimes (ca. 2.10 Ga). On the other hand, provenances, depositional ages and tectonic settings of low-grade Paleoproterozoic metasedimentary units in the NCC can also provide rigorous constraints on the tectonic evolution in the period between 2.45 and 2.00 Ga. In the Henan-Shaanxi province on the southern NCC, the Paleoproterozoic sedimentary sequences include the Shangtaihua Group, the Songshan Group, the Yinyugou Group, and the Tietonggou Formation. The low-grade metasedimentary Songshan Group deposited after 2.35 Ga and before 1.78 Ga and sourced from felsic rocks including major 3.00–2.40 Ga TTG gneisses, 2.40–1.95 Ga granitoid plutons and meta-rhyolites of the Dengfeng, Zhongtiao and Taihua complexes in the southern NCC, and minor 3.70–3.00 Ga transported exotic Paleoarchean and Mesoarchean crustal materials because no lithologies or zircons with such age founded in study areas. The Tietonggou Formation deposited at 1.91–1.80 Ga, with detrital zircon age peak of ~2.10 Ga which possibly sourced from ~2.10 Ga lithologic units in the south of NCC. The depositional ages of the Paleoproterozoic low-grade metasedimentary units in the NCC are constrained at a certain period of 2.35–1.78 Ga, which overlaps with the stage of subduction-collision related 2.45–2.20 Ga magmatisms and rift setting related 2.20–2.00 Ga magmatisms. The detrital zircon Hf isotopes of the low-grade sediments varied mainly toward the reduction of the radiogenic Hf isotope and gradually show a similar trend of the isotope trajectories of crustal evolution. Like previous studies, all these groups were deposited in basin settings which were not simple long-lived foreland basins. Combining with 2.45–2.00 Ga igneous rocks, they may evolved from back-arc or intra-arc basins developing at the subduction-collision stage (from ∼2.45 Ga) to rift stage (from ∼2.20 Ga) and then to foreland basins at the collision stage (from ∼1.85 to ∼1.80 Ga).

Main Tectonic Events and Metallogeny of the North China Craton

Precambrian period is an oldest and longest eon from 545 Ma to about 4500 Ma, taking over *90 % of the Earth’s history. The 80–90 % continental crust in the Earth generated in Precambrian and records complicated geotectonic processes. The most important geological events in Precambrian tectonic evolution include Neoarchean enormous crustal growth, tectonic regime inversion from pre-plate tectonics to plate tectonics, and Great Oxidention Event (GOE) in Paleoproterozoic. The North China Craton (NCC) is one of oldest cratons in the world and records almost all the important geological events occurred in other cratons of the Earth. The NCC also demonstrates some special characteristics, such as multi-stage cratonization, Paleoproterozoic rift–subduction–collision event, Earth’s paleo-climate and paleo-environment change, and Late Paleoproterozoic–Neoproterozoic multi-stage rifting event. These important geological events controlled mineralization with tectonic evolution and formed various and abundant ore deposits in the NCC. Here, we summarize geological events and metallogenic systems of the NCC and conclude that from Early Precambrian through Late Precambrian to Paleozoic and Mesozoic, the NCC records a transition from primitiveto modern-style plate tectonics. The NCCwent through five major tectonic cycles: (1) the Neoarchean crustal growth and stabilization, (2) Paleoproterozoic rifting–subduction–accretion–collision with imprints of the Great Oxidation Event (GOE), (3) Late Paleoproterozoic–Neoproterozoic multi-stage rifting, (4) Paleozoic orogenesis at the margins of the craton; and (5) Mesozoic extensional tectonics associated with lithosphere thinning and decratonization. Coinciding with these major geological events, five major metallogenic systems are identified as follows: (1) Archean BIF system, (2) Paleoproterozoic Cu–Pb–Zn and Mg-B systems, (3) Mesoproterozoic REE–Fe–Pb–Zn system, (4) Paleozoic orogenic Cu-Mo system, and (5) Mesozoic intracontinental Au and Ag–Pb–Zn and Mo systems. The ore deposit types in each of these metallogenic systems show distinct characteristics and tectonic affinities. The NCC provides one of the best examples to address secular changes in geological history and metallogenic epochs in the evolving Earth. This regular pattern is suitable to other continent blocks in the world, which reveals coevolution and irreversible character of Earth system on material, structure, and environment.

Late Mesozoic magmatism and tectonic evolution in the Southern margin of the North China Craton

Late Mesozoic granitic magmatism (158–112 Ma) are widespread in the southern margin of the North China Craton (NCC), contemporary with many world-class Mo-Au-Ag-Pb-Zn polymetallic deposits. There are abrupt changes in the elements and isotopic compositions of these granites at about 127 Ma. The early stage (158–128 Ma) granites show slightly or no negative Eu anomalies, large ion lithophile elements enriched and heavy REE depleted (such as Y and Yb), belonging to typical I-type granite. The late stage (126–112 Ma) granites are characterized by A-type and/or highly fractionated I-type granite, with higher contents of SiO2, K2O, Y, Yb and Rb/Sr ratio and lower contents of Sr, δEu value and Sr/Y ratio than that of the early-stage granites. Moreover, the whole rock Nd and Hf isotopic compositions of the granites younger than 127 Ma show more depleted than those of the older one. The two stages of Late Mesozoic granites were derived from a source region of the ancient basement of the southern margin of the NCC incorporated the mantle material. The late stage (126–112 Ma) granites contain more fractions of mantle material with depleted isotopic composition than the early ones. The granites record evidence for a strong crust-mantle interaction. They formed in an intracontinental extensional setting which was related to lithospheric thinning and asthenospheric upwelling in this region, which was possibly caused by westward subduction of the Paleo-Pacific plate. 127 Ma is an critical period of the transformation of the tectonic regime.

Petrogenesis and Tectonic Significance of the Late Paleoproterozoic to Early Mesoproterozoic (~1.80–1.53 Ga) A-Type Granites in the Southern Margin of the North China Craton

A-type granites are extremely rare before middle Paleoproterozoic but more frequent since late Paleoproterozoic globally, probably indicating a causal change of tectonics and deep geodynamic mechanism of the continents. The late Paleoproterozoic to early Mesoproterozoic tectonic transition is witnessed by the occurrence of the A-type granites in the southern margin of the North China Craton (NCC), including the 1.80 Ga Guijiayu and Motianzhai granites, 1.74 Ga Shicheng granite, 1.60 Ga Longwangzhuang and Maping granites, and 1.53 Ga Zhangjiaping granite. Most of the granites contain perthite, annite, and calcic amphibole but are lack of alkali mafic minerals. All of the granitic rocks show characteristics of typical A-type granites, with high silicon and total alkali, and HFSEs (i.e., Zr, Nb, Ce, Y) and low MgO, CaO, and P2O5; with negative Eu, Sr, and Ti anomalies and high FeOt/(FeOt + MgO) and Ga/Al ratios. Their geochemical characteristics, along with the negative zircon e Hf(t) and whole-rock e Nd(t) values, indicate that those A-type granites were derived from partial melting of ancient continental crust. Integrated with regional data, the late Paleoproterozoic to early Mesoproterozoic A-type granites in the southern margin of the NCC are most likely formed under post-collisional (1.80–1.78 Ga) to rifting regimes (1.78–1.53 Ga, or even younger), as Zhai and Peng (Acta Petrol Sinica, 23: 2665–2682, 2007) and Zhai et al. (Earth Sci Frontiers 21: 100–119, 2014, Tectonophysics, in press, 2015) proposed.

The Origin of Granulite Facies Metamorphic Iron Formations in Wuyang, Henan Province, Southern of North China Craton

The protoliths of Banded iron formations (BIF) formed as marine chemical sediments and can convey crucial information on the early history of mantle, tectonic, oceanic and biospheric processes(Bekker et al., 2010). The characteristics of the granulite facies metamorphic iron formations from the Wuyang region, Henan province, southern of North China Craton (NCC), is coexisting clinopyroxene and magnetite in iron-rich bands, even orthoand clinopyroxene, which provides a good mineral assemblage to estimate the metamorphic temperature. Wuyang iron formations (WIF) hosted in Neoarchean Tieshanmiao Formation of Taihua Group and consist of banded and massive structure in the same ore body with ambiguous contact within the open pits.