Cretaceous Magmatic Rocks Research Articles

New ages of Early Cretaceous magmatic rocks in the Yanbian area (NE China): implications for the subduction and slab rollback of the Paleo-Pacific Plate beneath eastern China during Early Cretaceous

ABSTRACT The precise age of slab rollback of the Paleo-Pacific Plate during Early Cretaceous has been contentious. In this study, we have conducted LA-ICP-MS zircon U-Pb dating, major and trace elements, and Sr-Nd isotopes analyses on the late Early Cretaceous diabase porphyries, andesitic basalts and basanites in the Yanbian area (NE China). Zircon U-Pb dating of basanites, andesitic basalts, and diabase porphyries yielded weighted average ages of 104 ± 1.9 Ma (MSWD = 0.55, n = 5), 106 ± 1.6 Ma (MSWD = 0.68, n = 6), and 111 ± 2 Ma (MSWD = 16, n = 3), respectively. The whole-rock geochemical results indicate that the late Early Cretaceous rocks predominantly belong to the low-K tholeiitic and medium-K calc-alkaline series. The diabase porphyries and andesitic basalts are characterized by large ion lithophile elements (e.g. Ba, K, and Sr) enrichment and high field strength elements (e.g. Nb and Zr) depletion with weak positive Eu anomalies, low initial 87Sr/86Sr ratios (0.70361–0.70394) and high ε Nd (t) values (5.96–7.87). The tectonic discrimination diagrams show that the diabase porphyries and andesitic basalts formed in the continental Island arc environment related to subduction of the Paleo-Pacific Plate in the late Early Cretaceous. In contrast, the basanites are enriched in Ba, Th, and U and depleted in K, Sr, and Ti, with weak negative Eu anomalies. The basanites display high magnesium values (Mg# = 0.79), which are approaching the primary magma. They have low Ti values (TiO2 = 1.45%) and Al values (A12O3 = 9.98–10.15%) and high Co values (78.1–80.2 ppm) and Ni values (549.5–585.5 ppm). The basanites formed in the intraplate extensional tectonic setting and were the products of lithospheric thinning and mantle upwelling during the late Early Cretaceous. The spatial-temporal relationship of late Early Cretaceous magmatic rocks in NE China suggests a slab rollback from high-angle subduction to low-angle subduction of the Paleo-Pacific Plate at ca. 106 Ma.

From the southern Gangdese Yeba arc to the Bangong-Nujiang Ocean: Provenance of the Upper Jurassic-Lower Cretaceous Lagongtang Formation (northern Lhasa, Tibet)

The stratigraphic records of sedimentary basins situated on the Lhasa block provide crucial evidence to reconstruct the early history of the Tibetan Plateau associated with the successive closure of oceanic branches and related magmatic activities. The Upper Jurassic to Lower Cretaceous Lagongtang Formation of the Nagqu Basin located in the northern part of the Lhasa block chiefly consists of fine-grained quartzose to litho-quartzose sandstones, siltstones and shales deposited in continental shelf to bathyal environments. Provenance data, including sandstone petrography, U-Pb ages and in situ Hf isotopes of detrital zircon, together with paleocurrent indicators, suggest that the Lagongtang Formation was mainly fed by recycling of quartz-rich siliciclastic strata of the central-southern Lhasa block. Fresh felsitic volcanic rock fragments, plagioclase prevailing among detrital feldspars, and zircon grains yielding the same age as that indicated by biostratigraphic analysis testify to penecontemporaneous magmatic activity. Volcanic detritus may have been derived from the Middle Triassic-Lower Cretaceous Yeba and/or Sangri volcanics exposed in the southern Lhasa block to the south as well as from Middle Triassic-Lower Cretaceous magmatic rocks of the central Lhasa block. Although the Nagqu Basin is adjacent to the Bangong–Nujiang suture, no debris from Bangong–Nujiang suture and Qiangtang was found in the Lagongtang Formation here. Our provenance data rule out the hypothesis that the Lagongtang Formation could have been deposited in a forearc basin associated with the southward subduction of the Bangong-Nujiang Ocean or in a basin related to the supposed Shiquanhe-Namco Ocean.

Evidences from zircon U-Pb geochronology, whole-rock geochemistry and Hf isotope of Early Cretaceous magmatic rocks in Baishan area, southern Jilin Province: Constraints on destruction of the North China Craton

对华北克拉通东北部早白垩世岩浆岩开展锆石U-Pb年代学、岩石地球化学及Hf同位素研究，能约束其岩石成因与构造背景，并为深入探讨晚中生代华北克拉通东北部岩石圈形成与演化提供依据。吉南白山地区的兴林花岗斑岩、红土崖安山质晶屑凝灰岩和江源含角砾流纹质岩屑晶屑凝灰岩的锆石为岩浆成因，结晶年龄分别为130&#177;1Ma、128&#177;1Ma和120&#177;1Ma。兴林花岗斑岩和红土崖安山质晶屑凝灰岩具高SiO₂、Al₂O₃、Sr含量及高Na₂O/K₂O，Sr/Y和（La/Yb）_N比值，贫Y、Yb，富集LILE （如Ba、Th、U），亏损HFSE （Nb、Ti、P），具有微弱的负铕异常（<i>δ</i>Eu=0.82~0.94），具埃达克质岩石特征。兴林花岗斑岩富碱，低TiO₂含量，铝饱和指数A/CNK=1.06~1.14，表现为右倾型稀土配分模式，全岩锆饱和温度为798~815℃，属I型花岗岩。兴林花岗斑岩<i>ε</i>_Hf（<i>t</i>）介于-17.96~-0.19，两阶段模式年龄<i>t</i>_DM2=1197~2313Ma，指示兴林花岗斑岩源自古元古代-中元古代的加厚镁铁质下地壳部分熔融。红土崖安山质晶屑凝灰岩<i>ε</i>_Hf（<i>t</i>）介于3.37~5.47，两阶段模式年龄<i>t</i>_DM2=830~967Ma，暗示红土崖安山质晶屑凝灰岩是新元古代拆沉榴辉岩质下地壳部分熔融并交代地幔橄榄岩的产物。结合江源含角砾流纹质岩屑晶屑凝灰岩<i>ε</i>_Hf（<i>t</i>）（-2.69~1.00），两阶段模式年龄<i>t</i>_DM2=1115~1349Ma，可知吉南地区早白垩世岩浆岩<i>ε</i>_Hf（<i>t</i>）具有不均一性，岩浆源区存在多组分的特点，既有古老地壳，也存在新生地壳，指示多阶段地壳生长过程，暗示华北克拉通东北部陆壳在元古代演化过程中存在两次增生事件。综合研究表明，早白垩世古太平洋板块（伊泽奈崎）的俯冲回撤导致增厚的华北克拉通东北部地壳整体处于伸展的构造背景下，并自西向东发生拆沉减薄，拆沉下地壳的部分熔融形成区域广泛分布的埃达克质岩石。

Early cretaceous tectonic setting of eastern Australia: Evidence from the subduction-related Morton Igneous Association of Southeast Queensland

Early Cretaceous magmatic rocks of the central east coast of Australia are recognized as a lithologically, geochemically, and geochronologically distinctive rock package herein named the Morton Igneous Association. The association embraces small plutons and hypabyssal intrusions within the Jurassic and Lower Cretaceous Nambour and Maryborough basins. It also includes volcanics of the Grahams Creek Formation, basal infill of the Maryborough Basin with an indicated volume exceeding 0.25 M km3, representing a much more substantial igneous assemblage. Plutonic rocks vary from hornblende-rich gabbro to granodiorite, and syenite. The Grahams Creek Formation consists of volcanic to volcaniclastic rocks of basaltic to dacitic composition. The association is UPb zircon dated at 145–140 Ma, and is characterized by distinctive subduction-related trace element compositions typical of magmatic rocks from continental arc settings. Oxygen isotope and εHf compositions obtained from zircon are consistent with a largely mantle source for the host magmas. Trace element data for the 130–95 Ma Whitsunday Igneous Province, of more northerly location on the east Australian coast, are consistent with a continental back-arc association. The MIA and Whitsunday Igneous Province represent small parts of an extensive Cretaceous active margin system developed for east Gondwana, mainly located on continental crust comprising northern Zealandia which is now separated from east Australia, largely submerged and little documented. East Australia records both older (145–140 Ma) continental arc magmatism and younger (132–95 Ma) continental back-arc magmatism attributed to progressive Cretaceous roll-back of a consuming plate boundary to east Gondwana.

Southeastern continuation of the Bangong-Nujiang suture zone: Constraints from Middle Jurassic–Early Cretaceous sedimentary rocks in the western Baoshan block, SW China

The Bangong-Nujiang Ocean (BNO) separated the Lhasa and Qiangtang blocks in Tibet during the Mesozoic. Whether it separated the Tengchong and Baoshan blocks and how it evolved remain debated. The sedimentary rocks from the Middle Jurassic Mengjia and Liuwan formations, Late Jurassic Longhai Formation, and Early Cretaceous Longkan Formation of the western Baoshan block serve to investigate these issues. Geochemical signatures indicate that their sediments were deposited on a passive continental margin, with a collisional provenance identified in the Late Jurassic–Early Cretaceous formations. All the formations show similar detrital zircon U–Pb age groups with the exceptions of the Longhai Formation having minor 190–150 Ma zircons and the Longkan Formation containing abundant 190–150 and 140–120 Ma zircons. Provenance studies indicate that the sediments of the Middle Jurassic formations were sourced from the southern Qiangtang, Baoshan and Simao blocks and the Changning–Menglian suture zone. In contrast, the sediments of the Late Jurassic–Early Cretaceous formations were derived mainly from the Tengchong and Baoshan blocks, with an increasing contribution of the Tengchong-derived materials from Late Jurassic to Early Cretaceous. We infer that the absent, minor and significant contributions from Tengchong-derived sources in the provenance of the Baoshan strata were associated with the BNO that separated the Tengchong and Baoshan blocks during the Middle Jurassic, the initiation of the Tengchong-Baoshan collision in the Late Jurassic, and that the Tengchong and Baoshan blocks had been welded together in the Early Cretaceous, respectively. Therefore, the widely spreaded and subduction-related Early Cretaceous magmatic rocks in the Tengchong block were produced by the eastward subduction of the Neo-Tethyan Ocean.

Late Cretaceous volcanism and fluid circulation in the South Atlantic: Insights from continental carbonates in the onshore Namibe Basin (Angola)

The Namibe Basin developed in the Early Cretaceous as part of the South Atlantic rift system between Africa and South America. Magmatic activity occurred during the syn-rift (Valanginian-Hauterivian) and post-rift (Coniacian-Early Campanian) phases, with the latter triggering fluid circulation events that had significant impacts on reservoir quality and hydrocarbon prospectivity. The continental carbonates of the Mariquita Member (Mb.) developed during the Late Cretaceous magmatic pulse and record the associated fluid circulation. Thus, study of these carbonates provides insights into paleo-fluid sources and circulation pathways, which can ultimately help to constrain diagenetic phases, their effects on reservoir quality, and hydrocarbon re-mobilisation.This study analyses the tectonostratigraphic setting, facies and diagenesis of the Mariquita Mb. carbonates by integrating field, petrographic and geochemical data. The carbonates are stratigraphically bound by Upper Cretaceous magmatic rocks, and constitute a series of fault-associated spring mound systems that pass laterally into lacustrine sediments. The main diagenetic events include pervasive dolomitisation, silicification and multi-stage fracturing. Fluid temperatures during dolomitisation were between ∼32 and 56 °C, while silicification occurred between ∼70 and > 260 °C. Fluids likely followed fault-controlled convective circulation pathways. The fluid compositions comprised infiltrated/modified meteoric and marine waters, as suggested by negative carbon isotopes, enriched oxygen and strontium isotopes, and REE data. Fluid modification occurred via interaction with subsurface rock units, such as basement, volcanic/siliciclastic aquifers and evaporites. However, higher temperatures during silicification could suggest late contributions from magmatic fluids too. Bitumen within porosity possibly reflects regional hydrocarbon mobilisation processes driven by magmatic fluids.

Mesozoic multiple magmatic events in Bangor area: Constraints on the tectonic evolution of Bangong–Nujiang Tethys Ocean

The Bangor area is located in the middle part of the Bangong–Nujiang Suture Zone (BNSZ), where a large number of Middle Jurassic‐Early Cretaceous magmatic rocks are exposed, and is a key area for studying the tectonic evolution of the Bangong–Nujiang Tethys Ocean (BNO). However, the arc magmatic system triggered by the southward subduction of the BNO has not been systematically identified in the northern margin of the Lhasa Terrane, and the timing of initial subduction and closure is still controversial. In this study, four granite, two amphibolite, and one gabbro samples from the Bangor area yield two clusters intrusive ages of 162–168 Ma and 121–133 Ma. The Stages I and II granites show whole‐rock geochemistry similar to those of continent arc magmatism, characterized by enrichment in large ionic lithophile elements (LILEs; e.g., Th, Rb, K) and depletion in high field strength elements (HFSEs; e.g., Nb, Ta, P, Ti). The Stage I magmatic rocks consist of granodiorite, amphibolite, and gabbro and have similar zircon εHf(t) values of −10.1 to −3.5, which may be formed by partial melting of enriched mantle wedge related to the southward subduction of the BNO. The Stage II highly differentiated I‐type granites show positive zircon εHf(t) values of +1.5 to +9.9, which are considered the product of highly differentiated processes via the mixed magma derived from depleted mantle and lower crustal rocks. The Stage II I‐type granites show negative zircon εHf(t) values of −9.2 to −0.7, which are derived from a mixed magma source generated by partial melting of lower crust and enriched lithospheric mantle. On the basis of data from this study and previous research studies, a four‐stage tectonic evolutionary model of the Bangong–Nujiang Tethys Ocean is constructed: Stage 1 (190–145 Ma), the northward and southward subduction of BNO formed a series of arc magmatism; Stage 2 (145–135 Ma), a magmatism lull occurred on the north side due to the subduction effect of the ocean plateau, while the southward subduction still continued; Stage 3 (135–116 Ma), the roll‐back of subducted slab enhanced the magmatism in this stage; and Stage 4 (&lt;116 Ma), the closure of BNO lead to the formation of syn‐collisional magmatism.

Early Cretaceous magmatism of the Southern Qiangtang terrane and its tectonic significance

The tectonic evolution of the Bangong-Nujiang Tethys is known as the most important event in the late Mesozoic in the central Qianghai-Tibet Plateau. Magmatism is a probe and window for detecting deep crust, which also records the process of tectonic evolution. The magmatic arc formed at Early Cretaceous in the Southern Qiangtang subterrane, was products of the Mesozoic magmatism during the long-term subduction of the Bangong-Nujiang Tethys oceanic lithosphere and the subsequent Qiangtang-Lhasa terrane collision. This article summarizes zircon U-Pb geochronology, major and trace elements, and Sr-Nd-Pb-Hf isotopic data for the Early Cretaceous magmatic rocks located in the southern Qiangtang subterrane of central Tibet. However, there are three hypotheses about the tectonic background of the Early Cretaceous magmatism of the South Qiangtang subterrane: (1) oceanic ridge subduction; (2) slab roll-back; (3) slab break-off.

Cretaceous magmatic rocks in the Nyima area, North Tibet: Constraints for the tectonic evolution of the Bangong-Nujiang suture zone

Cretaceous magmatic rocks in the Nyima area, North Tibet: Constraints for the tectonic evolution of the Bangong-Nujiang suture zone

Transition from oceanic subduction to continental collision in central Tibet: evidence from the Cretaceous magmatism in Qiangtang block

ABSTRACT The Mid-Late Mesozoic magmatic arc in southern Qiangtang block was associated with the long-lasting subduction of the Bangong-Nujiang Tethyan and the subsequent Lhasa-Qiangtang collision. The detailed processes associated with the transition from oceanic lithosphere subduction to lithosphere delamination remain unclear. This paper reports zircon U-Pb age, major and trace element and Sr-Nd isotopic data for the Cretaceous two-phase magmatic rocks in the Shuanghu region of the southern Qiangtang block located in central Tibet. Zircon dating on the Qushenla volcanic rocks yields a 206Pb/238U age of 114 ± 4 Ma. The Abushan volcanic rocks comprise rhyolites. Zircon dating on two rhyolites yields a 206Pb/238U age of 74 ± 1 to 76 ± 1 Ma. In contrast with the Qushenla volcanic rocks, the Abushan rhyolites exhibit features typical of high-fractionated I-type granitoids, high K calc-alkaline and shoshonite, and peraluminous, suggesting that the Abushan rhyolites formed in post-collision setting. The Qushenla volcanic rocks have slightly higher initial 87Sr/86Sr (0.70466–0.70537), and εNd(t) (1.71–2.89) in comparison with the Abushan rhyolites (87Sr/86Sr = 0.70453–0.70491, εNd(t) = 1.63–2.35). The Qushenla volcanic rocks were most likely derived from the mixing of mantle-derived components and crustal material. And the crust assimilation and fractional crystallization played a role during the ascent of magma. The Abushan volcanic rocks derived from magmatic mixing between the crustal materials and mantle-derived components, and undergoing extensive fractional crystallization. Taking into account the regional tectonic and magmatic data, we suggest that the transition from the oceanic lithosphere subduction to lithosphere delamination occurred during the Late Cretaceous in central Tibet. It evolved through three stages: 125–100 Ma, late subduction of Bangong-Nujiang Tethys; 100–87 Ma slab break-off of subduction Bangong-Nujiang Tethys; and 87–73 Ma lithosphere delamination.

Simultaneous growth and reworking of the Lhasa basement: A case study from Early Cretaceous magmatism in the north-central Tibet

Magmatic rocks preserve a lot of information about the formation and evolution of continental crust, and they play an important role in understanding the growth and reworking of ancient continent. Cretaceous magmatic rocks occurring in the Lhasa terrane are a window to investigate magmatic processes and continental evolution of the Tibetan Plateau. In this study, we report zircon UPb ages and Hf isotopic data, as well as whole-rock major and trace elemental compositions for Early Cretaceous magmatic rocks, including andesite and dacite from the Zenong Group, and associated diorite and granite in the Xinji area, north-central Tibetan Plateau. Zircon from andesite and dacite yielded weighted-mean 206Pb/238U ages of 120–108 Ma and variable zircon εHf(t) values between −12.3 and +2.2. Geochemical compositions suggested that they both displayed high-K calc-alkaline features. Andesite was probably formed by mixing between crust- and mantle-derived melts, whereas dacite was most likely generated by partial melting of ancient crust with input of mantle-derived material. Dioritic and granitic intrusions were intruded into the Precambrian strata and Zenong Group, respectively. Zircon from diorite gave a weighted-mean 206Pb/238U age of ca. 116 Ma and zircon εHf(t) values of −0.5 to +7.3. Diorite displayed medium-K calc-alkaline series signatures and was most likely formed by partial melting of mantle wedge peridotite accompanied by crustal assimilation. Granite yielded zircon weighted-mean 206Pb/238U ages of 107–104 Ma, and positive zircon εHf(t) values of +1.5 to +6.7, indicating a major source of juvenile crust. Granitic samples show irregular REE distribution with variable Ce anomalies, which were probably inherited from subducted pelagic sediments. The Early Cretaceous magmatic rocks in the Xinji area records distinct formation processes involved partial melting of mantle, re-melting of crust, and mixing between mantle and crust. Such processes, however, had a complicated source from mantle, ancient and juvenile continental crust, and recycled pelagic sediments, suggesting simultaneous crustal growth and reworking of the Lhasa basement during the Early Cretaceous.

Zircon U-Pb ages, geochemistry, and Sr-Nd isotope ratios for early cretaceous magmatic rocks, southern Saqqez, northwestern Iran

Sanandaj-Sirjan Zone (SaSZ) is one of the most dynamic structural zones of Iran, which is divided into three main parts: Northern, Central and Southern. The northern SaSZ has been affected by deformation due to fault activities near the Zagros suture zone, and mylonitic structures have overprinted these rocks and was affected by three episodes of magma injection during the Permian-Carboniferous, Early Cretaceous and Cenozoic. In this study, the rock units investigated that have been considered Precambrian-Paleozoic basement on geological maps. This paper considers zircon U-Pb dating, whole-rock chemistry and Sr-Nd isotope ratios of Cretaceous magmatic rocks in the N-SaSZ to develop a new geodynamic model for the evolution of these magmatic rocks. The new zircon U-Pb ages obtained in this study show that the magmatic rocks crystallized at 115–107 Ma in the Early Cretaceous (Aptian-Albian) and are much younger than the supposed ages presented on geological maps. This complex classified into two main groups of basic-intermediate and acidic rocks based on SiO2 contents. The whole-rock chemistry of the basaltic and andesitic rocks, which are interbedded with marine shallow-water sedimentary deposits, shows their typical calc-alkaline affinity and subordinate tholeiitic series on an active margin. The positive εNd(t) of approximately +4 for some undifferentiated basalts with negative Ti and Nb anomalies shows the relation of these rocks to calc-alkaline magmatism and was generated by the partial melting of subcontinental lithospheric mantle (SCLM). Granitoid rocks with some affinity to the peraluminous group with a negative εNd(t) value (-3.2) mainly and negative Ti and Nb anomalies plot in an active margin tectonic setting. Simultaneous mafic calc-alkaline volcanism and the generation of granitic intrusions in the Early Cretaceous could have occurred on an active margin. Due to the absence of Jurassic arc related magmatic rocks in northern SaSZ and presence of Cretaceous calc alkaline magmatic activity, which are not observed in the central SaSZ, support the idea that the subduction of the Neotethys beneath the northern SaSZ started in the Early Cretaceous.

U–Pb zircon age and geochemistry of the Cuocun gabbro in the southern Lhasa Terrane: Implications for Early Cretaceous rollback of the Neo‐Tethyan oceanic slab

The geodynamic mechanism of the Early Cretaceous magmatic rocks in the southern Lhasa Terrane remains controversial. To constrain the Early Cretaceous petrogenesis and tectonic evolution of the Neo‐Tethyan oceanic slab, we have undertaken zircon U–Pb dating, whole‐rock major and trace element analyses, and Sr–Nd–Pb–Hf isotopic studies of the Cuocun gabbro from the southern Lhasa Terrane. The zircon U–Pb dating yielded an Early Cretaceous crystallization age of 146.9 ± 0.6 Ma (MSWD = 0.77, 2σ). The Cuocun gabbro is enriched in large‐ion lithophile elements (e.g., Rb, Sr, and Ba) and depleted in high‐field‐strength elements (e.g., Nb, Ta, and Ti). The initial 87Sr/86Sr value are low (0.70361–0.70460), εNd(t) and εHf(t) values are high and positive (from +5.1 to +6.6 and from +11.0 to 17.0, respectively), and Pb isotope specific value are homogeneous (initial 206Pb/204Pb = 18.163–18.457, 207Pb/204Pb = 15.541–15.632, and 208Pb/204Pb = 38.262–38.673). The gabbro originated from the partial melting of a depleted mantle wedge that had reacted with slab‐derived fluids, and the ascending magmas underwent fractional crystallization without significant crustal contamination. Combining our data with those from previous studies of Early Cretaceous magmatism in the southern Lhasa Terrane, we suggest that the Early Cretaceous magmatic rocks (147–130 Ma) in the southern Lhasa Terrane were generated within a continental marginal arc setting related to the rollback of the northwards subducting Neo‐Tethyan oceanic slab.

Zircon U-Pb dating of the Lower Cretaceous strata on the margin of the Erlian Basin, North China and its Constraints on basin evolution

A large number of Early Cretaceous magmatic rocks are exposed in the Erlian Basin in North China. LA-MC-ICP-MS zircon U-Pb dating of rhyolites with age of 122.8 ± 0.4 Ma is corresponded to basin subsidence, reflecting a large-scaled extension event occurred in North China. There was a structural inversion after the Neogene, which caused the migration of the Erlian Basin center from the NW to SE, with the center of the Early Cretaceous basin corresponding with the NW margin of the modern basin.

Late Cretaceous adakitic rocks from the western Tibetan Plateau: implications for the subduction of the Neo-Tethys Ocean

ABSTRACT It is still hotly debated whether the subduction of the Indus-Yarlung Zangbo Tethyan oceanic lithosphere led to Cretaceous magmatism in the north Lhasa terrane or not. Here, we provide new data on adakitic dacite in the Rutog area, western Tibetan Plateau. Zircon U-Pb dating and Lu-Hf analyses yielded a weighted-mean 206Pb/238U age of ca. 97 Ma for a dacite sample, and corresponding positive zircon εHf(t) values of +5.3 to +10.1. All samples have high Sr (796–1091ppm), but low Y (10.4–11.9 ppm) and Yb (0.68–0.79 ppm) contents, with pronounced high Sr/Y (72.5–97.6) ratios. Furthermore, they have low K2O (1.11–1.49 wt.%) and Th (4.18–4.54 ppm) contents, and low K2O/Na2O (0.18–0.28) and Th/La (0.22–0.24) ratios. These geochemical features are most simialr to those of adakite derived from partial melting of subducted oceanic slab. These new data, in combination with regional geology, enable us to suggest that the Late Cretaceous arc-affinity magmatic rocks were probably triggered by northward subduction of the Indus-Yarlung Zangbo oceanic lithosphere. This will give new insights into the geodynamic evolution of the Indus-Yarlung Zangbo Neo-Tethys Ocean.

Mantle influx compensates crustal thinning beneath the Cathaysia Block, South China: Evidence from SINOPROBE reflection profiling

The crustal architecture of the Cathaysia Block, South China, is a key element in understanding the late Mesozoic tectono-magmatic history of East Asia, and includes a spectacular (∼260,000 km2) basin and igneous province that forms a natural laboratory for unravelling the interplay of extension, magmatism and crust-mantle interaction. Here we analyze the whole crustal architecture of East Cathaysia through a ∼300-km-long SINOPROBE deep seismic reflection profile. This profile reveals that East Cathaysia is underlain by two seismically distinct terranes (i.e., the Wuyishan and Coastal terranes), separated by a low-angle, northwest-dipping set of mantle-penetrating reflectors interpreted as the Zhenghe-Dapu fault zone, which documents Neoproterozoic terrane amalgamation and Cretaceous extensional reactivation. The seismic image of the Coastal terrane shows large-scale low reflectivity truncated by arrays of high-reflective subhorizontal bands of layered reflectors, corresponding to an extension-related crustal architecture dominated by extensive Cretaceous magmatism. The layered reflectors are interpreted to represent mantle-derived mafic sills, which document significant magmatic underplating associated with asthenospheric upwelling beneath the terrane. This interpretation accounts for the localized distribution of Cretaceous magmatic rocks in the Coastal terrane. We suggest that the magmatic underplating added large volumes of mantle-derived material to the crust, compensating for the crustal thinning. Such a magma-compensating process might have formed an integral part of crustal reworking and growth in creating the magma-dominated crust and the broadly flat Moho of the Coastal terrane during the Cretaceous.

Geochemical Study of Cretaceous Magmatic Rocks and Related Ores of the Hucunnan Cu–Mo Deposit: Implications for Petrogenesis and Poly-Metal Mineralization in the Tongling Ore-Cluster Region

The Hucunnan porphyry- and skarn-type Cu–Mo deposit is located in the south of the central Shizishan ore field of the Tongling ore-cluster region. The intrusive Hucunnan granodiorite, outcropping in this deposit, has adakitic geochemical features, and its magma is proposed to have originated from partial melting of the oceanic crust mixed with mantle-derived materials. The porphyry-type orebody is hosted in the granodiorite, whereas the skarn-type orebody occurs in the contact zones of intrusions and country rocks. The δ34S values of pyrite from the skarn orebodies ranged from +3.9 to +4.7‰ (avg. +4.3‰, n = 6), while those of the porphyry orebodies ranged from +5.1 to +6.2‰ (avg. +5.6‰, n = 4). 208Pb/204Pb, 207Pb/204Pb, and 206Pb/204Pb ratios of the pyrites from the skarn orebodies were 38.04–38.45 (avg. 38.26), 15.55–15.66 (avg. 15.59), and 18.16–18.54 (avg. 18.44), respectively (n = 6). The pyrites in the porphyry orebodies had 208Pb/204Pb, 207Pb/204Pb, and 206Pb/204Pb ratios of 38.24–38.36, 15.51–15.662, and 18.10–18.41, respectively (avg. 38.32, 15.58, 18.22; n = 4), respectively. The metallogenic model ages from Re–Os isotopic dating were 138.7 ± 1.9 and 140.0 ± 2.8 Ma, respectively. Geochemical data indicate that the ore-forming fluids in the skarn stage are characterized by high temperature, low acidity, and high oxygen fugacity, and the ore-forming materials were mainly from magma and partly from stratum, proving that the skarn orebody has more stratum materials than the porphyry orebody.

Subduction Polarity in Ancient Arcs: A Call to Integrate Geology and Geophysics to Decipher the Mesozoic Tectonic History of the Northern Cordillera of North America: REPLY

Subduction Polarity in Ancient Arcs: A Call to Integrate Geology and Geophysics to Decipher the Mesozoic Tectonic History of the Northern Cordillera of North America: REPLY

Transition from oceanic subduction to continental collision recorded in the Bangong-Nujiang suture zone: Insights from Early Cretaceous magmatic rocks in the north-central Tibet

The transition from oceanic subduction to continental collision is a key stage in the evolution of ancient orogens. We present new data for Early Cretaceous diorite and granite porphyry from north–central Tibet to constrain the evolution of the Bangong–Nujiang Tethyan Ocean (BNTO). The diorites have moderate SiO2 and high MgO contents, similar to high-Mg andesites. Zircon grains yield U–Pb ages of 128–124 Ma and positive εHf(t) values between +13.2 and + 16.3, corresponding to Hf depleted-mantle model ages (TDM) of 281–131 Ma. The high-Mg diorite was probably formed by partial melting of hydrous mantle wedge fluxed by slab-derived fluids in an oceanic subduction setting. The granite porphyries yield zircon U–Pb ages of 117–115 Ma and zircon εHf(t) values ranging from +0.1 to +4.5. Most samples have high SiO2 and Fe2O3T contents, variable FeOT/MgO and Ga/Al ratios, and are depleted in Ba, Sr, P, and Ti, similar to I- and A-type granites. The granite porphyries were most likely derived from partial melting of juvenile dioritic or granodioritic crust due to break-off of the BNTO lithosphere following collision between the Lhasa and Qiangtang blocks. The Early Cretaceous high-Mg diorite and A-type granite porphyry thus record the Early Cretaceous transition from oceanic subduction to continental collision along the Bangong–Nujiang suture zone (BNSZ).

Petrological investigation of Late Cretaceous magmatism in Kaboodan area, NE Iran: Evidence for an active continental arc at Sabzevar zone

New zircon U-Pb dating indicates widespread Cretaceous magmatism in the Kaboodan area which includes two distinct geological zones of Sabzevar and Taknar at the NE of Iran. The Cretaceous igneous rocks were formed during 99–75 Ma and are consist of various extrusive and intrusive groups. All igneous rocks of Kaboodan have geochemical characteristics indicative of an active continental arc setting and there is no genetic linkage between the extrusive and intrusive rocks. The extrusive rocks appear at the Sabzevar zone and consist of basaltic andesite, andesite, dacite, and different pyroclastic units. The basaltic andesite samples are the most primitive extrusive rocks with low 87Sr/86Sri (∼0.704) and high ɛNdt values (> +6) while other samples show variable isotopic ratios (87Sr/86Sri = 0.704–0.708 and ɛNdt = +1.41 to +4.25).The Cretaceous intrusive rocks appear in both of Taknar and Sabzevar zone sections of the Kaboodan area. The intrusive rocks of the Sabzevar zone section have granitoid composition while Taknar Zone Cretaceous bodies are bimodal and consist of major mafic and minor felsic components. The mafic rocks include hornblende gabbros (ca. 99 Ma) with an arc-related composition similar to high aluminum basalts and Sr-Nd isotopic values (87Sr/86Sri = 0.7037 and ɛNdt = 4.06–4.87) which imply that they have originated from the hydrous magma by relatively deep partial melting of the mantle sources. All Cretaceous granitoid rocks were crystallized at ca. 81 Ma and depict similar isotope ratios (87Sr/86Sri = 0.7045–0.705, ɛNdt = +1.84 to +2.61) but other geochemical characteristics are different among the granitoids of Sabzevar zone, Taknar zone granitoids, and dike granitoids. Geochemical composition of the Late Cretaceous magmatic rocks at the Kaboodan area suggest that they were generated in an active continental arc by different partial melting from similar or heterogeneous mantle-derived sources in association with crystal fractionation.