Tectonic evolution of Cretaceous extensional basins in Zhejiang Province,eastern South China: structural and geochronological constraints

Widespread Cretaceous volcanic basins are common in eastern South China and are crucial to understanding how the Circum-Pacific and Tethyan plate boundaries evolved and interacted with one another in controlling the tectonic evolution of South China. Lithostratigraphic units in these basins are grouped, in ascending order, into the Early Cretaceous volcanic suite (K_1V), the Yongkang Group (K_1-2), and the Jinqu Group (K₂). SHRIMP U-Pb zircon geochronological results indicate that (1) the Early Cretaceous volcanic suite (K_1V) erupted at 136–129 Ma, (2) the Yongkang Group (K_1-2) was deposited from 129 Ma to 91 Ma, and (3) the deposition of the Jinqu Group (K₂) post-dated 91 Ma. Structural analyses of fault-slip data from these rock units delineate a four-stage tectonic evolution of the basins during Cretaceous to Palaeogene time. The first stage (Early to middle Cretaceous time, 136–91 Ma) was dominated by NW–SE extension, as manifested by voluminous volcanism, initial opening of NE-trending basins, and deposition of the Yongkang Group. This extension was followed during Late Cretaceous time by NW–SE compression that inverted previous rift basins. During the third stage in Late Cretaceous time, possibly since 78.5 Ma, the tectonic stress changed to N–S extension, which led to basin opening and deposition of the Jinqu Group along E-trending faults. This extension probably lasted until early Palaeogene time and was terminated by the latest NE–SW compressional deformation that caused basin inversion again. Geodynamically, the NW–SE-oriented stress fields were associated with plate kinematics along the Circum-Pacific plate boundary, and the extension–compression alternation is interpreted as resulting from variations of the subducted slab dynamics. A drastic change in the tectonic stress field from NW–SE to N–S implies that the Pacific subduction-dominated back-arc extension and shortening were completed in the Late Cretaceous, and simultaneously, that Neo-Tethyan subduction became dominant and exerted a new force on South China. The ongoing Neo-Tethyan subduction might provide plausible geodynamic interpretations for the Late Cretaceous N–S extension-dominated basin rifting, and the subsequent Cenozoic India–Asia collision might explain the early Palaeogene NE–SW compression-dominated basin inversion.     

Late Cretaceous tectono-magmatic activity in the Nize region,central Tibet: evidence for lithospheric delamination beneath the Qiangtang–Lhasa collision zone

ABSTRACT

The results of zircon U–Pb age dating and whole-rock geochemistry for the Late Cretaceous Nize granodiorite porphyries, combined with analysis of near-coeval structural deformation of the Lower Cretaceous Langshan Formation, provide new data to better understand the tectonic evolution of the northern Lhasa subterrane, central Tibet. Zircon U–Pb ages of 89.2 ± 0.3 Ma to 87.8 ± 0.3 Ma indicate emplacement during the Late Cretaceous. Granodiorite porphyry intrusions were contemporaneous with the development of a regional angular unconformity, overlain by the Upper Cretaceous Jingzhushan (or Abushan) Formation, within the collision zone between the South Qiangtang and Lhasa terranes. Geochemical data for Nize granodiorite porphyries indicate that they have a calc-alkaline composition enriched in large-ion lithophile elements and light rare earth elements and depleted in high-field-strength elements and heavy rare earth elements. High Al₂O₃ and Sr contents, low Yb and Y contents, and high Sr/Y ratios are similar to adakitic magmas.

Structural analysis indicates two stages of deformation (D₁ and D₂), with D₁ forming the focus of the present study. The D₁ deformation is represented by large-scale faults and records two periods of faulting. These periods are recognized as early compressional thrust faulting and a dominant late stage characterized by normal faulting and extension, with the latter stages of D₁ being near-coeval with the emplacement of the Nize granodiorite porphyries. The combination of zircon ages, geochemical data, and structural analysis indicates that the Nize granodiorite porphyries formed after collision of the South Qiangtang and Lhasa terranes. Adakitic magma derived from partial melting of the thickened lower or middle crust resulted from lithospheric delamination that may have been promoted by the convective removal of deeper lithospheric mantle.     

Cretaceous deformation history of the middle Tan-Lu fault zone in Shandong Province, eastern China

Based on field analysis of fault-slip data from different rock units of the Cretaceous basins along the middle part of the Tan-Lu fault zone (Shandong Province, eastern China), we document polyphase tectonic stress fields and address the changes in sense of motion of the Tan-Lu fault zone during the Cretaceous. The Cretaceous deformation history of the Tan-Lu fault zone can be divided into four main stages. The first stage, during the earliest Cretaceous, was dominated by N-S extension responsible for the formation of the Jiaolai basin. We interpret this extension to be related to dextral strike-slip pull-apart opening guided by the Tan-Lu fault zone. The second stage, during the middle Early Cretaceous, was overwhelmingly rift-dominated and characterized by widespread silicic to intermediate volcanism, normal faulting and basin subsidence. It was at this stage that the Tan-Lu-parallel Yi-Shu Rift was initiated by E-W to WNW-ESE extension. The tectonic regime then changed during the late Early Cretaceous to NW-SE-oriented transpression, causing inversion of the Early Cretaceous rift basin and sinistral slip along the Tan-Lu fault zone. During the Late Cretaceous, dextral activation of the Tan-Lu fault zone resulted in pull-apart opening of the Zhucheng basin, which was subsequently deformed by NE-SW compression. This deformation chronology of the Tan-Lu fault zone and the associated Cretaceous basins allow us to constrain the regional kinematic models as related to subduction along the eastern margin of Asia, or related to collision in the Tibet region.     

Systematic changes in orientation of linear mylonitic fabrics: An example of strain partitioning during transpressional deformation in north Golpaygan,Sanandaj–Sirjan zone,Iran

Kilometer-scale, shallowly dipping, NW-striking top-to-the NE reverse and dextral strike-slip shear zones occur in metamorphic rocks of north Golpaygan. These metamorphic rocks are exposed at the NE margin of the central part of the Sanandaj–Sirjan zone in the hinterland of the Zagros orogen. NW-striking top-to-the NE normal shear zones were also found in a small part of the study area. Structural evidence of three deformation stages were found. Pre-mylonitization metamorphic mineral growth happened during D₁. The main mylonitization event was during the D₂ deformational event, following coaxial refolding, synchronous to retrograde metamorphism of amphibolite to greenschist facies in the Late Cretaceous–Paleocene, and before D₃ folding and related mylonitization. We documented the systematic changes in the orientations of D₂ linear fabrics especially stretching lineations and superimposition relations of structures. It is concluded that the dextral strike-slip and dip-slip shear zones were coeval kinematic domains of partitioned dextral transpression. The shallowly dipping reverse and strike-slip shear zones are compatible with partitioning in a very inclined transpressional model. Fabric relations reflect that the top-to-the NE normal shear zones were not produced during deformation partitioning of inclined dextral transpression. The Late Cretaceous–Paleocene strain partitioning was followed by later N–S shortening and NE-extension in the north Golpaygan area.     

The Late Jurassic extensional event in the central part of the South China Block – evidence from the Laoshan’ao shear zone and Xiangdong Tungsten deposit (Hunan,SE China)

ABSTRACT

The Late Mesozoic tectonics strongly reworked the tectonic framework of East Asia. In the South China Block (SCB), the major Late Mesozoic phenomena are featured by numerous magmatic activities, (half-) graben basins opening, and abundant ore deposits formation. The intrinsic relationships of these phenomena with the tectonic regime are still hotly debated, partly due to the lack of structural data. To advance the understanding of these issues, we conducted a detailed structural analysis including field and microscopic observations as well as an Anisotropy of Magnetic Susceptibility (AMS) investigation in the Laoshan’ao shear zone (LSA), which is the northern branch of the major Chaling–Chenzhou–Linwu Fault (CCLF) in the SCB. The new data enable us to reach the following conclusions: the evolution of the LSA can be divided into three deformation phases. D₁ is a NW–SE-stretching event featured by the SE-dipping normal fault with a top-to-the-SE shear sense, coeval with the emplacement of the 154 Ma Batuan syn-tectonic pluton that leads to the development of the Xiangdong tungsten deposit and the opening of the Cretaceous Chaling basin; D₂ is a NE–SW-striking dextral strike–slip event that dislocated the ore veins formed during the D₁ event; D₃ is a NW-directed thrusting event that cross-cut the previous gneissic foliation. D₁ is the major event of the LSA shear zone, interpreted as the reactivation of the CCLF in response to the Late Mesozoic tectonics in the SCB, and indicates a NW-SE extensional regime since the Late Jurassic in the study area. This study also provides an example for an ore-forming process controlled by both the hydrothermal fluid coming from a syn-tectonic granite and the accommodation of tension gashes opened by the regional extensional event.     

Nature and origin of Mesozoic granitoids and associated mineralization in the Sanjiang Tethys Orogeny,SW China: the Xiuwacu complex example

ABSTRACT

The magmatic generation for the Late Triassic–Early Jurassic (~215–200 Ma) and Early Cretaceous–Late Cretaceous (~108–79 Ma) post-collisional granites in the Sanjiang Tethys orogeny remain enigmatic. The Xiuwacu complex, located in the southern Yidun Terrane, consists of biotite granite with a weight mean ²⁰⁶Pb/²³⁸U age of 199.8 ± 2.5 Ma, aplite granite of 108.2 ± 2.3 Ma, monzogranite porphyry of 80.8 ± 1.0 Ma, and diorite enclaves of 79.2 ± 0.9 Ma and 77.9 ± 0.8 Ma. The Late Triassic biotite granites show I-type granite affinities, with high SiO₂ contents, high Mg^# values, high zircon δ¹⁸O values, and negative whole-rock ?Nd(t) values, indicating a predominant ancient crustal source with the input of juvenile materials. Their fractionated REE patterns and concave-upward middle-to-heavy REE patterns require garnet-bearing amphibolite as the melt source. The Cretaceous highly fractionated aplite granites and monzogranite porphyries have relatively high SiO₂ contents, high (Na₂O + K₂O)/CaO ratios, high zircon δ¹⁸O values, and enriched whole-rock Sr–Nd isotopic signatures, suggesting that their parent magmas were likely originated from the ancient middle- to lower crust. Their significant negative Eu anomalies and obvious depletions in Nb, Sr, and Ti demonstrate that the Cretaceous granitic magmas had experienced more fractionation than the Late Triassic felsic magmas. The Late Cretaceous diorite enclaves show low SiO₂ contents, high Mg^# values, and high zircon δ¹⁸O values, suggesting that they were probably derived from the partial melting of subcontinental lithospheric mantle enriched by the Late Triassic subduction. The Late Triassic–Early Jurassic and Early Cretaceous–Late Cretaceous magmatism witnessed the post-collisional setting and intraplate extensional setting in response to the slab break-off and lithospheric-scale transtensional faulting, respectively. The partial melting of subduction-modified lithospheric mantle or/and residual sulphide cumulates within the lower crust during the origination of Late Cretaceous magmas could have provided metals for the formation of Xiuwacu deposit.     

大别山东北缘桐城高压-超高压变质带的构造解析及其对郯庐断裂带的制约

通过对郯庐断裂带南段桐城地区高压-超高压变质带详细的岩石学和构造学研究,将研究区从空间结构上划分为三个构造单元:上部低温-高压单元、中部中温-高压单元和下部超高压单元。根据研究区多期构造变形分析,共识别出了五期有区域构造地质学含义的事件(D_1-D_5):D_1代表高压-超高压变质岩中-晚三叠世同碰撞早期折返过程;D_2表征了高压-超高压变质岩晚三叠世同碰撞晚期折返过程;D_3记录了早白垩世中大别变质核杂岩的形成,也即整个中国东部晚中生代大规模伸展构造在研究区的表现;D_4可能标志着郯庐断裂走滑构造对高压-超高压造山带的叠加;D_5表现为脆性正断作用,控制了晚白垩世-古近纪潜山半地堑盆地的形成。这些结果表明了研究区所经历构造演化的复杂性,其构造几何形态很难用郯庐断裂左行平移南大别超高压变质岩来解释,也不支持桐城地区存在巨大走滑作用的证据。     

Tectonometamorphic evolution of the Permo-Triassic Songrim (Indosinian) orogeny: evidence from the late Paleozoic Pyeongan Supergroup in the northeastern Taebaeksan Basin,South Korea

The deformed and metamorphosed middle Pennsylvanian to Early Triassic (?) rocks of the Pyeongan Supergroup, occurring in northeastern flank of the Paleozoic Taebaeksan Basin, east central South Korea, record the impact of the Late Permian–Triassic Songrim (Indosinian) collisional orogeny in the eastern Eurasia margin. Analysis of meso- and microstructures and their relationship with porphyroblast growth has revealed that these metasedimentary rocks were affected by four deformation events (D₁–D₄). During the polyphase deformations, successive foliations with alternately steep (S₁ and S₃) and flat-lying (S₂ and S₄) orientations were produced with growth of chloritoid, andalusite and kyanite porphyroblasts during D₁ and D₂, and growth of garnet, staurolite and andalusite porphyroblasts during D₃ and D₄. SHRIMP U–Pb detrital zircon ages (330–310 Ma) from metapelites in the oldest unit of the Pyeongan Supergroup in the study area, combined with the previous data on sandstone compositions, indicate magmatic activity that was contemporaneous with the initial deposition of the Pyeongan Supergroup. This also suggests that the Pyeongan Supergroup was deposited in an arc-related foreland basin at an active continental margin during the Late Carboniferous and Permian time. Two contractional deformation events (D₁ and D₃) and two events related to gravitational flattening (D₂ and D₄), along with Barrovian-type medium-pressure regional metamorphism, may have resulted from the Late Permian–Triassic Songrim orogeny, related to collision between the North and South China blocks and/or collision between the Eastern Eurasia margin and the proto-Japan terrane.     

Sedimentary provenance of the Hengyang and Mayang basins,SE China,and implications for the Mesozoic topographic change in South China Craton: Evidence from detrital zircon geochronology

Detrital zircon U–Pb data from sedimentary rocks in the Hengyang and Mayang basins, SE China reveal a change in basin provenance during or after Early Cretaceous. The results imply a provenance of the sediment from the North China Craton and Dabie Orogen for the Upper Triassic to Middle Jurassic sandstones and from the Indosinian granitic plutons in the South China Craton for the Lower Cretaceous sandstones. The 90–120 Ma age group in the Upper Cretaceous sandstones in the Hengyang Basin is correlated with Cretaceous volcanism along the southeastern margin of South China, suggesting a coastal mountain belt have existed during the Late Cretaceous. The sediment provenance of the basins and topographic evolution revealed by the geochronological data in this study are consistent with a Mesozoic tectonic setting from Early Mesozoic intra-continental compression through late Mesozoic Pacific Plate subduction in SE China.     

台湾玉里带变质岩LA-ICP-MS锆石U-Pb年龄及其地质意义

台湾玉里带位于台湾东部,其主体为变质砂岩、千枚状泥岩夹绿帘片岩、阳起片岩、浅变质枕状玄武岩等(原岩为海相火山-陆源沉积岩),其次为高压相的蓝闪片岩。火山岩属钙碱性系列,具低Ba、Nb、Sr和高Pb丰度,稀土元素总量普遍较低,具有轻稀土元素弱富集、Eu无亏损的稀土元素配分模式;(含火山碎屑)变质砂岩的LA-ICP-MS锆石U-Pb年龄表明,~(206)Pb/~(238)U表面年龄有6个峰值:最新年龄23.3Ma(古近纪)为火山碎屑锆石年龄,也是成岩年龄;中生代—古生代年龄有96.7Ma(晚白垩世)、124.2Ma及130.6Ma(早白垩世)、214.1~228Ma(晚三叠世)、283.7Ma和289Ma(早二叠世)等,锆石为次棱角状或次圆状,代表了源岩的岩浆年龄;~(207)Pb/~(206)Pb表面年龄出现古元古代峰值(1769~1852Ma)和新太古代峰值(2506Ma和2530Ma),均为磨圆的碎屑变质或岩浆锆石,代表了不明古元古代变质基底的年龄。玉里带变质岩原岩可能形成于古近纪(23.3Ma),与南海扩张的白云运动同时,属白云运动有关的火山作用的产物。     

江南东段地区NE-SW向断裂带断层滑移矢量反演及其大地构造意义

华南中-新生代构造演化受太平洋构造域和特提斯洋构造域的联合控制.以江南东段NE-SW向景德镇-歙县剪切带和球川-萧山断裂中发育的脆性断层为研究对象,利用野外交切关系和断层滑移矢量反演方法厘定了7期构造变形序列并反演了各期古构造应力场,讨论了断层活动的时代及其动力学.白垩纪至新生代研究区7期古构造应力场分别为:(1)早白垩世早期(136～125Ma)NW-SE向伸展;(2)早白垩世晚期(125～107Ma)N-S向挤压和E-W向伸展;(3)早白垩世末期至晚白垩世早期(105～86Ma)NW-SE向伸展;(4)白垩世中期(86～80Ma)NW-SE向挤压和NE-SW向伸展;(5)晚白垩世晚期至始新世末期(80～36Ma)N-S向伸展;(6)始新世末期至渐新世早期(36～30Ma)NE-SW向挤压和NW-SE向伸展;(7)渐新世早期至中新世中期(30～17Ma)NE-SW向伸展.结合区域地质研究表明,第1期至第4期古构造应力场与古太平洋构造域的板片后撤、俯冲以及微块体(菲律宾地块)间的碰撞作用有关;第5期伸展作用受控于新特提斯构造域俯冲板片后撤,而第6期和第7期古构造应力场主要与印-亚碰撞的远程效应有关.白垩纪至新生代,华南东部受伸展构造体制和走滑构造体制的交替控制.先存断裂的发育可能是导致华南晚中生代走滑构造体制的主要控制因素.     

Significant Zn–Pb–Cu remobilization of a syngenetic stratabound deposit during regional metamorphism: A case study in the giant Dongshengmiao deposit,northern China

The giant Dongshengmiao Zn–Pb–Cu deposit is located in the Langshan district, northern China. The ores are hosted within a Proterozoic rift sequence, which underwent lower greenschist facies metamorphism and shear deformation during development of Early Cretaceous intraplate orogenic belt. Northwest-dipping thrust faults, which share similar orientations and dip angles with the orebodies, are well developed in the mining area. Syngenetic stratabound sulfides were formed during the Proterozoic rifting event, but syngenetic ore textures have seldom been preserved except for some pretectonic fine-grained pyrite. Petrological observation, ³⁹Ar/⁴⁰Ar geochronology, combined with previous isotopic and fluid inclusion studies indicates that significant Zn–Pb–Cu remobilization took place as a result of thrust faulting associated with metamorphic devolatilization of ore-hosting rocks at ca. 136 Ma, coeval with the intraplate orogeny and regional crustal shortening. Sulfides were redistributed in shear structures or along grain boundaries of ore-hosting carbonates, and Fe-rich carbonates were ideal sites for Zn–Pb–Cu precipitation.     

Middle Jurassic–Early Cretaceous tectonic evolution of the Bayanhushuo area,southern Great Xing’an Range,NE China: constraints from zircon U–Pb geochronological and geochemical data of volcanic and subvolcanic rocks

This study presents new zircon U–Pb geochronology, geochemistry, and zircon Hf isotopic data of volcanic and subvolcanic rocks that crop out in the Bayanhushuo area of the southern Great Xing’an Range (GXR) of NE China. These data provide insights into the tectonic evolution of this area during the late Mesozoic and constrain the evolution of the Mongol–Okhotsk Ocean. Combining these new ages with previously published data suggests that the late Mesozoic volcanism occurred in two distinct episodes: Early–Middle Jurassic (176–173 Ma) and Late Jurassic–Early Cretaceous (151–138 Ma). The Early–Middle Jurassic dacite porphyry belongs to high-K calc-alkaline series, showing the features of I-type igneous rock. This unit has zircon ε_Hf(t) values from +4.06 to +11.62 that yield two-stage model ages (T_DM2) from 959 to 481 Ma. The geochemistry of the dacite porphyry is indicative of formation in a volcanic arc tectonic setting, and it is derived from a primary magma generated by the partial melting of juvenile mafic crustal material. The Late Jurassic–Early Cretaceous volcanic rocks belong to high-K calc-alkaline or shoshonite series and have A₂-type affinities. These volcanics have ε_Hf(t) and T_DM2 values from +5.00 to +8.93 and from 879 to 627 Ma, respectively. The geochemistry of these Late Jurassic–Early Cretaceous volcanic rocks is indicative of formation in a post-collisional extensional environment, and they formed from primary magmas generated by the partial melting of juvenile mafic lower crust. The discovery of late Mesozoic volcanic and subvolcanic rocks within the southern GXR indicates that this region was in volcanic arc and extensional tectonic settings during the Early–Middle Jurassic and the Late Jurassic–Early Cretaceous, respectively. This indicates that the Mongol–Okhotsk oceanic plate was undergoing subduction during the Early–Middle Jurassic, and this ocean adjacent to the GXR may have closed by the Late Middle Jurassic–Early Late Jurassic.     

Cretaceous sedimentary basins in Sichuan,SW China: Restoration of tectonic and depositional environments

This study documents sediment infill features and their responses to the tectonic evolution of the Sichuan Basin and adjacent areas. The data include a comparison of field outcrops, well drillings, inter-well correlations, seismic data, isopach maps, and the spatial evolution of sedimentary facies. We divided the evolutionary history of the Sichuan Cretaceous Basin into three stages based on the following tectonic subsidence curves: the early Early Cretaceous (145–125 Ma), late Early Cretaceous to early Late Cretaceous (125–89.8 Ma), and late Late Cretaceous (89.8–66 Ma). The basin underwent NW–SE compression with northwestward shortening in the early Early Cretaceous and was dominated by alluvial fans and fluviolacustrine sedimentary systems. The central and northern areas of the Sichuan Basin were rapidly uplifted during the late Early Cretaceous to early Late Cretaceous with southwestward tilting, which resulted in the formation of a depression, exhibited southwestward compression, and was characterized by aeolian desert and fluviolacustrine deposits. The tectonic framework is controlled by the inherited basement structure and the formation of NE mountains, which not only affected the clastic supply of the sedimentary basin but also blocked warm-wet currents from the southeast, which changed the climatic conditions in the late Late Cretaceous. The formation and evolution of Cretaceous sedimentary basins are closely related to synchronous subtle far-field tectonism and changes in climate and drainage systems. According to the analysis of the migration of the Cretaceous sedimentation centers, different basin structures formed during different periods, including periods of peripheral mountain asynchronous thrusting and regional differential uplift. Thus, the Sichuan Cretaceous sedimentary basin is recognized as a superimposed foreland basin.     

The Late Cretaceous source-to-sink system at the eastern margin of the Tibetan Plateau: Insights from the provenance of the Lanping Basin

The surface uplift of the Tibetan Plateau(TP) and its geomorphology evolution has triggered aridification of Asia's interior and drainage development at the eastern margin of the plateau.However, how the pre-Cenozoic early growth histories of the TP impact the drainage system and climate is poorly constrained.The Late Mesozoic Lacustrine evaporite-bearing basins on the eastern margin of the TP record significant information on the uplift of the source terranes, source-to-sink system development and climate change.In this study, we presented detrital zircon U–Pb ages from the Upper Cretaceous Yunlong Formation in the Lanping Basin, as well as Hf isotopic, petrographic, direct statistical, and multidimensional scaling analyses, and use them to characterize the provenance and reconstruct the drainage system.All of the samples have five major age peaks at 200–290 Ma, 400–490 Ma,750–1000 Ma, 1750–1950 Ma, and 2400–2600 Ma with mostly negative ε_(Hf)(t) values(81%).We infer the sediments are primarily derived from recycled sediments of the Songpan-Garze terrane, and partly from the Sichuan Basin and the Southern Qiangtang terrane, as well as the exposed magmatic rocks of the Yidun Arc and SongpanGarze terrane.The provenance features of the contemporaneous sediments from the Sichuan, Xichang, Chuxiong,and Simao basins indicate a complex hierarchical drainage pattern on the eastern margin of the TP during the Late Cretaceous.The hierarchical drainage system exhibits a complete gradational cycle of lake-basin types from overfilled freshwater Sichuan Basin through balanced fill saline Xichang Basin and underfilled hypersaline Chuxiong, Lanping, Simao, and Khorat Plateau basins from proximal to distal.The early growth of the TP primarily controlled the drainage and lake-basin evolution by not only causing the uplift and exhumation of the source areas and providing large amounts of clastic material to the proximal sub-drainage areas but also intensifying the aridity and deposition of evaporites.     

Geochronology,geochemistry and tectonic significance of the late Mesozoic volcanic sequences in the northern Wuyi Mountain volcanic belt of South China

The widespread occurrence of late Mesozoic volcanic rocks in the Gan-Hang Belt in South China is associated with similarly widespread mineralization, but many important questions surrounding these volcanic rocks have not been clearly answered. The Tianhuashan basin located in the northern Wuyi Mountain volcanic belt is one of the most important volcanic basins in the Gan-Hang Belt, and it is primarily composed of the Daguding and Ehuling Formations and their intrusive counterparts. LA-ICP-MS zircon U–Pb dating shows that the Daguding Formation erupted in the Late Jurassic (152–160 Ma), whereas the Ehuling Formation erupted in the Early Cretaceous (131–139 Ma) in the Tianhuashan basin. Volcanic rocks are rhyolite and share similar trace and rare earth element patterns with an enrichment of LREEs and a depletion in Sr, Ba, Nb, Ta, P, Eu and Ti. They are also characterized by negative whole rock ε_Nd(t) and zircon ε_Hf(t) values with Paleoproterozoic t_2DM ages, suggesting that they were derived primarily from the remelting of ancient crustal materials. Daguding volcanic rocks are strongly peraluminous and show a higher Mg# than pure crustal melts, implying that they were likely derived from Paleoproterozoic metasedimentary basement materials. However, Ehuling volcanic rocks are weakly peraluminous and have a pronounced A₂-type geochemical signature. Detailed elemental and isotopic data suggest that they were formed by the partial melting of the Paleoproterozoic metamorphic basement (including metasedimentary and metaigneous rocks) at a high temperature (~ 840 °C), followed by fractional crystallization. These results imply that during the Late Jurassic, South China on the Gan-Hang Belt was a continental arc coupled with the subduction of the Paleo-Pacific plate. Since the beginning of the Early Cretaceous, an intra-arc rift has formed along the Gan-Hang Belt as a consequence of slab rollback. These results also indicate that the extension in the Gan-Hang Belt began later than the southwestern part of the Shi-Hang Zone and lasted from 139 Ma to 122 Ma.     

Formation of intra-arc volcanosedimentary basins in the western flank of the central Peruvian Andes during Late Cretaceous oblique subduction: field evidence and constraints from U–Pb ages and Hf isotopes

During late Early to Late Cretaceous, the Peruvian coastal margin underwent fast and oblique subduction and was characterized by important arc plutonism (the Peruvian Coastal Batholith) and formation of volcanosedimentary basins known as the Western Peruvian Trough (WPT). We present high-precision U–Pb ages and initial Hf isotopic compositions of zircon from conformable volcanic and crosscutting intrusive rocks within submarine volcanosedimentary strata of the WPT hosting the Perubar massive sulfide deposit. Zircons extracted from both the volcanic and intrusive rocks yield concordant U–Pb ages ranging from 67.89±0.18 Ma to 69.71±0.18 Ma, indicating that basin subsidence, submarine volcanism and plutonic activity occurred in close spatial and temporal relationship within the Andean magmatic arc during the Late Cretaceous. Field observations, satellite image interpretation, and plate reconstructions, suggest that dextral wrenching movements along crustal lineaments were related to oblique subduction. Wrench tectonics is therefore considered to be the trigger for the formation of the WPT as a series of pull-apart basins and for the emplacement of the Coastal Batholith. The zircon initial _Hf values of the dated magmatic rocks fall between 5.5 and 7.4, and indicate only very subordinate influence of a sedimentary or continental component. The absence of inherited cores in the zircons suggest a complete lack of old basement below the WPT, in agreement with previous U–Pb and Sr isotopic data for batholithic rocks emplaced in the WPT area. This is supported by the presence of a most likely continuous block of dense (~3.0 g/cm³) material observed beneath the WPT area on gravimetric crustal cross sections. We suggest that this gravimetric anomaly may correspond to a piece of lithospheric mantle and/or oceanic crust inherited from a possible Late Permian–Triassic rifting. Such young and mafic crust was the most probable source for arc magmatism in the WPT area.     

Late Jurassic I-type rhyolites from the Duobuza region,north-central Tibet: evidence for the occurrence of juvenile lower crust and crustal growth in the Southern Qiangtang Terrane

ABSTRACT

Previous studies of the Southern Qiangtang (SQ) Terrane in Tibet have shown significant growth of continental crust during the late Mesozoic (ca. 170–120 Ma) through underplating of mantle-derived melts beneath ancient lower crust. However, the mechanism of crustal growth remains defective during the Late Jurassic. In this contribution, we report new zircon U–Pb ages, whole-rock major and trace element concentrations, and zircon Hf isotopic compositions of Late Jurassic rhyolites from the Duobuza area in the SQ Terrane. Zircon LA–ICP–MS U–Pb dating yields concordant ages and weighted mean ²⁰⁶Pb/²³⁸U ages of 151–149 Ma, indicating that the rhyolites were erupted during the Late Jurassic. The rhyolite samples yield high SiO₂ (69.1–73.4 wt.%) and Na₂O (3.82–6.49 wt.%) concentrations, high values of the differentiation index (95.4–98.5), moderate Al₂O₃ (13.1–15.5 wt.%) and K₂O (2.55–3.26 wt.%) concentrations, and relatively low TiO₂ (0.34–0.44 wt.%), MgO (0.14–0.67 wt.%), and MnO (0.01–0.10 wt.%) concentrations and Mg# values (8.9–33.5). They are enriched in Rb, Th, Ba, and Pb, depleted in Nb, Ta, Sr, Ti, and P, and yield negative Eu anomalies. These characteristics suggest that the Duobuza rhyolites represent medium- to high-K calc-alkaline, highly fractionated I-type granitoids. Variations in major and trace element concentrations indicate that the rhyolites underwent significant fractionation of Fe–Ti oxides, plagioclase, K-feldspar, hornblende, biotite, and apatite. The samples yield positive zircon ε_Hf(t) values (+0.08 to +10.9) and young T^CD _M model ages (816–386 Ma), indicating that they were generated through partial melting of juvenile lower crust of the SQ Terrane. These data are distinct from those of Late Jurassic intermediate–felsic plutons in the SQ Terrane that generally yield negative ε_Hf(t) values and ancient T_C ^DM model ages. We suggest that the occurrence of the Duobuza rhyolites is the significant witness. The presence of juvenile lower crust beneath the SQ Terrane during the Late Jurassic (ca. 150 Ma) rather than during the Early Cretaceous (ca. 120Ma). The transition from ancient to juvenile lower crust has already began in the SQ Terrane during the Late Jurassic.     

Cretaceous A-type volcanic–intrusive rocks and simultaneous mafic rocks along the Gan-Hang Tectonic Belt,Southeast China: petrogenesis and implications for the transition of crust–mantle interaction

ABSTRACT

Abundant evidence points to the Cretaceous crust–mantle interaction and plate subduction in the Gan-Hang Tectonic Belt (GHTB), southeastern China, but the evolutionary process remains poorly constrained. Here we conduct a comprehensive study on Daqiaowu granitic porphyry and diabase dikes in the eastern GHTB, in conjunction with previous studies on simultaneous felsic and mafic rocks along the GHTB, to demonstrate their petrogenesis and geodynamic evolutionary process. The Daqiaowu granitic porphyry (125 Ma), as well as the coeval granitic rocks, exhibits high zircon saturation temperatures, alkalis, 10⁴*Ga/Al ratios, and Zr + Nb + Ce + Y contents, concluding a distinctive belt of the Early Cretaceous (~137–125 Ma) A-type volcanic–intrusive rocks in the GHTB. Their ε_Nd(t) and zircon ε_Hf(t) values gradually increased through time from approximately ?9.0 to ?1.0 and ?10.0 to +4.0, respectively, implying increasing contribution of mantle-derived components to their formation, and hence progressively intensified crust–mantle interaction in an intra-arc rift environment (a geodynamic transition stage from continental arc to back-arc) during the Early Cretaceous. This plausibility is further supported by the Early Cretaceous Daqiaowu diabase dikes and coeval mafic rocks which exhibit arc-like magmatic signatures and were derived from mantle wedge. In contrast, the Late Cretaceous mafic rocks show ocean island basalt-like geochemical characteristics, reflecting a depleted asthenosphere mantle source. This discrepancy of mantle sources concludes that the geodynamic setting in the GHTB may have basically transferred to back-arc regime in the Late Cretaceous. Thus, the Cretaceous geodynamic evolutionary process in the GHTB can be defined as the Early Cretaceous gradually intensified crust–mantle interaction in a geodynamic transition stage (from continental arc to back-arc extension) and the Late Cretaceous back-arc extensional setting.     

扬子地块与南秦岭造山带的盆山系统与构造耦合

本文重新厘定了扬子地块西北缘晚古生代至早中生代沉积盆地的原型,在综合分析南秦岭造山带和勉略缝合带形成规律的基础上,对于南秦岭造山带与扬子地块北缘的拼合演化历史以及盆山耦合关系进行了研究。指出在晚二叠世晚期(长兴组沉积上段)和早三叠世早期(飞仙关组沉积下段)发生点式碰撞,在两个不同的大地构造单元之间形成了与碰撞相关的裂谷盆地群(包括开江-梁平裂谷、城口-鄂西裂谷和东部的当阳裂谷等),碰撞裂谷群的持续演化时间为5~6Ma,这一阶段典型的沉积标志为水下早期阶段形成的海相磨拉石层序。至早三叠世的嘉陵江二段沉积时期,两个不同地块的持续拼合导致大巴山和米苍山地区与周缘前陆盆地相关的古冲断带的形成,该阶段在缝合带接触部位发育角度不整合和河流相沉积,扬子地块其余大部仍然是保持连续的海相碳酸盐岩沉积。晚三叠世南秦岭造山带与扬子北缘之间的残余大洋消失,为整体闭合的碰撞后期阶段,沉积了须家河组开始的陆相碎屑岩系,大巴山和米苍山地区进入到了以陆相磨拉石为主的前陆盆地阶段,在扬子北缘形成了神农架-黄陵隆起和米苍山隆起。晚三叠世以后大巴山和米苍山地区进入了比较复杂的后期改造阶段,产生了多期的收缩性构造活动,包括以形成区域性的假整合和小角度不整合为特征的晚侏罗世-早白垩世早期(J₃-K₁)的低幅度活动期;以大巴山和米苍山冲断带的强烈改造为主,形成薄皮冲断构造系统的早白垩世晚期变形和以形成大巴山弧形冲断带和米苍山基底卷入的冲断带为特征的新生代晚期变形。