Mesozoic Igneous Rocks Research Articles

Slow‐Growing and Extended‐Duration Seismicity Swarms: Reactivating Joints or Foliations in the Cahuilla Valley Pluton, Central Peninsular Ranges, Southern California

AbstractThree prolific earthquake swarms and numerous smaller ones have occurred since 1980 in the Mesozoic igneous plutonic rocks of the Perris block of the Peninsular Ranges, Southern California. The major swarms occurred in 1980–1981, 1983–1984, and 2016–2018, with the latest swarm still ongoing. These swarms have no clear mainshock, with the largest events of ML 3.6, ML 3.7, and Mw 4.4. Each successive swarm had larger cumulative seismic moment release with about 314 and 411 events of M ≥ 1.5, while the third swarm has produced about 451 events of M ≥ 1.5 (as of September 2018). The concurrent strike‐slip faulting occurred on north to northwest striking planes but with no orthogonal northeast trending seismicity alignments. These shallow swarms are probably driven by intrablock Pacific‐North America plate boundary stress loading of the two bounding major late Quaternary strike‐slip faults, the Elsinore and San Jacinto faults. The state of stress within the Cahuilla Valley pluton has a ~40° angle between the maximum principal stress and the average trend of the swarms, suggesting that migrating pore fluid pressures aid in the formation and growth of zones of weakness. These swarms, which last more than 600 days each, exhibit clear bilateral spatial migration for distances of up to ~7–8 km and reach their full length in about 20 months. The slow spatial‐temporal development of the swarms corresponds to a fluid diffusivity of 0.006 to 0.01 m2/s, consistent with very low permeability rocks as expected for this block. There is no geodetic or other evidence for a slow slip event driving the swarms.

Geochemistry of Late Mesozoic volcanic rocks in the central Great Xing’an Range, NE China: petrogenesis and crustal growth in comparison with adjacent areas

ABSTRACTThe central Great Xing’an Range contains widespread exposures of Mesozoic igneous rocks, yet the origin of its Late Mesozoic volcanic rocks has long been controversial. They are composed of andesite, trachyte, trachyandesite, trachydacite, rhyolite, and minor basaltic andesite. The intermediate rocks are enriched in large ion lithophiles and light rare earth elements (LILEs and LREEs) and are depleted in high field strength elements (HFSEs) such as Nb, Ta, and Ti, with positive εHf(t) values (+6.2 to +13.4) and Hf single-stage model ages (TDM1 = 556–227 Ma). This indicates that they were derived from the partial melting of a mantle wedge that was modified by previously subducted slab-derived fluids was likely introduced with limited crustal contamination. The felsic rocks show positive εHf(t) values (+4.3 to +10.2) and Hf two-stage model ages (TDM2 = 910–544 Ma), with more enrichment of LILEs and LREEs and more depletion of HFSEs and negative Eu anomalies. This indicates that felsic rocks were mainly derived from partial melting of Neoproterozoic accreted quartz-amphibolite facies mafic crust. We summarise the geochronological and Hf-Nd isotope data for Mesozoic igneous rocks within the central Great Xing’an Range (CGXR), the northern Xing’an block (NXB), and the Erguna block (EB), with the aim of constraining the geochronological framework for Mesozoic volcanism and comparing the growth of continental crust. The evolution of Late Mesozoic magmatism in the three areas reached its climax ~125 Ma ago. Our data, together with regional isotope evidence, suggest that crustal accretion of the CGXR occurred during the Neoproterozoic–Phanerozoic. However, that of the EB occurred during the Meso- to Neoproterozoic. The EB exhibits more lateral and vertical heterogeneities and ancient continental crust than the CGXR and NXB in the northern magma source region.

Ages and petrogenesis of the Late Mesozoic igneous rocks associated with the Xiaokele porphyry Cu–Mo deposit, NE China and their geodynamic implications

The genesis of igneous rocks associated with the newly discovered Xiaokele porphyry Cu-Mo deposit, located in the eastern Erguna Block, NE China, can give insights into the regional geodynamic evolution during the Late Mesozoic. Several ore-bodies have been identified in a granodiorite porphyry stock, which intruded the rhyolite and was subsequently intruded by diorite porphyry and granite porphyry. The U–Pb ages of zircons from rhyolite, mineralized granodiorite porphyry, diorite porphyry and granite porphyry associated with the Xiaokele deposit are 152.5 ± 1.7, 150.0 ± 1.6, 147.9 ± 1.3, and 123.2 ± 1.7 Ma, respectively. The rhyolite and granite porphyry have high SiO2 and low MgO, Cr, Co, and Ni contents. Their isotopic data show negative εNd(t) values of −4.76 to −0.87, initial 87Sr/86Sr ratios of 0.7066 to 0.7134 and positive zircon εHf(t) values of 0.33–5.56, indicating that they were derived from a basaltic lower continental crust. The mineralized granodiorite porphyry is characterized by high Sr/Y values and low Y (3.7–8.0) contents, showing adakite affinity. The low K2O/Na2O (0.51–0.7) ratios, high CaO contents, weakly negative εNd(t) (−1.17 to −0.27), and low zircon εHf(t) (1.49–5.4) values from the adakitic samples indicate that they were derived from the partial melting of an altered oceanic slab together with assimilation of mantle peridotite and crustal materials. The 148 Ma diorite porphyry has high Mg# (51–58), weakly negative and positive εNd(t) (−0.09 to 0.01) values, and lower (87Sr/86Sr)i (0.7055–0.7057) ratios, suggesting an enriched mantle wedge source. Considering the late Mesozoic regional tectonic evolution of the Erguna Block and adjacent area, we propose that the formation of the Xiaokele deposit is linked to the southward flat-slab subduction of the Mongol–Okhotsk oceanic plate during the Later Jurassic-Early Cretaceous.

Using seismic geomorphology and detrital zircon geochronology to constrain provenance evolution and its response of Paleogene Enping Formation in the Baiyun Sag, Pearl River Mouth Basin, South China sea: Implications for paleo-Pearl River drainage evolution

The syn-rift sedimentary history of the Pearl River Mouth Basin (PRMB) in the northern South China Sea is poorly understood due to restricted borehole coverage. In this study, an integrated analysis of 3D seismic data, detrital zircon data and the data from previous provenance studies has been used to document the shifting of provenance and depositional response during the Paleogene Enping Formation of the Baiyun Sag in the PRMB. Our results show that the Baiyun Sag was filled mainly with sediments derived from two potential sources: the intrabasinal source of uplifts in the PRMB (Mesozoic igneous rocks) and the extrabasinal source of the South China Block (Precambrian–Paleozoic metamorphic and magmatic rocks). During the deposition of the lower and middle members of the Enping Formation (EpL–EpM), the sediment of the Baiyun Sag was derived primarily from the intrabasinal source. The sediment was transported over relatively short distance through faulted troughs and incised valleys and deposited in a series of small-to medium-scale braided deltas, beach bars and fan deltas. However, during the deposition of the upper member of the Enping Formation (EpU), the sediment of the Baiyun Sag was derived mainly from a mixture of extrabasinal and intrabasinal sources. The sediment of the EpU was transported over relatively long distance via large-scale incised valleys and deposited in three stages large-scale progradational braided delta. The detrital zircon age spectra of the EpU in the northwestern area show four well-recognized age clusters with peaks at ∼842 Ma, ∼450 Ma, ∼252 Ma and ∼151 Ma, which correspond to tectono-magmatism in the South China Block. The analysis of provenance shifting in this study may be of great value in constraining the tectonic and sedimentary evolution of the PRMB, northern South China Sea margin, as well as the drainage evolution of the paleo-Pearl River. Our findings can also provide potential benefits for the integrated analysis of geophysics and geochronology data through time in the petroliferous basins.

Final Closure of the Paleo‐Asian Ocean and Onset of Subduction of Paleo‐Pacific Ocean: Constraints From Early Mesozoic Magmatism in Central Southern Jilin Province, NE China

AbstractWhen the Paleo‐Asian Ocean closed and the Paleo‐Pacific subduction started in NE China remains controversial. To constrain these key issues, we carried out zircon U‐Pb‐Hf isotopic and whole‐rock geochemical studies of Early Mesozoic igneous rocks in central southern Jilin Province, NE China. Early Mesozoic igneous rocks in region were formed at three stages: the middle Triassic (246 Ma), the late Triassic (219–220 Ma), and the early Jurassic (172–194 Ma). Middle Triassic trachyandesites in southern Jilin Province were solely generated through partial melting of ancient lower crust of the North China Craton (NCC), implying ongoing southward subduction of Paleo‐Asian Ocean Plate beneath the NCC at this time. Late Triassic trachyandesites in southern Jilin Province were derived from the magma mixing/mingling of the NCC lithospheric mantle and juvenile eastern Central Asian Orogenic Belt crustal sources. Late Triassic hornblende gabbros in central Jilin Province were derived from depleted lithospheric mantle previously metasomatized by subduction‐related fluids. Coeval adakitic granodiorites were likely produced through the partial melting of thickened juvenile lower crust. Spatially, these rocks occurred on both sides of the Solonker‐Xra‐Moron‐Changchun Suture Zone and record N–S directed extension accompanied by the final closure of the Paleo‐Asian Ocean. Early Jurassic andesites were produced through mixing/mingling between partial melts derived from eastern Central Asian Orogenic Belt lithospheric mantle and ancient NCC crust. In contrast, Early Jurassic felsic rocks, including A‐type granites, were derived from the partial melting of Neoproterozoic accreted lower crust, jointly implying an extensional setting as the result of the initial subduction from Paleo‐Pacific Plate at this time.

Early Cretaceous adakitic lavas and A-type rhyolites in the Songliao Basin, NE China: Implications for the mechanism of lithospheric extension

There is a broad consensus that the extensive late Mesozoic igneous rocks in NE China were generated in an extensional setting. However, the timing and mechanism of the lithospheric extension remain controversial. To address this, we carried out an integrated study involving LA–ICP–MS zircon U–Pb dating and geochemical analyses (major elements, trace elements, and Hf isotopes) for the Early Cretaceous adakitic lavas and A-type rhyolites of the Songliao Basin. The adakitic lavas are andesites and dacites. The U–Pb dating of zircons from the adakitic lavas and A-type rhyolites yielded ages between 115 and 102 Ma. Geochemically, the adakitic lavas are characterized by high Sr contents (515–1610 ppm) and low Y (0.98–17.58 ppm) and heavy rare earth element (HREE) contents, and they therefore have high Sr/Y (51–112) ratios. They also exhibit high Mg# values (36–57) and high contents of MgO (0.56–3.53 wt%), Cr (15.7–87.3 ppm), and Ni (6.7–44.7 ppm) that are comparable with those of high-Mg adakitic rocks. The A-type rhyolites show an affinity with aluminous A-type magmatic rocks, and they are metaluminous to peraluminous (A/CNK = 0.98–1.35), enriched in alkalis, Ga, Zr, Nb, and Y, depleted in Sr and P, and exhibit fractionated REE patterns with negative Eu anomalies (Eu/Eu* = 0.05–0.77). All the primary zircons from the adakitic lavas and A-type rhyolites have positive εHf(t) values of +3.6 to +12.1 and juvenile two-stage model (TDM2) ages of 934–392 Ma. The adakitic lavas probably resulted from the partial melting of a delaminated region of the lower continental crust, with the magmas subsequently interacting with mantle materials upon ascent, while the A-type rhyolites were probably generated by the partial melting of a dehydrated charnockitic middle–lower crust. The data suggest that the adakitic lavas and the A-type rhyolites formed in an extensional environment related to the rollback of the subducting Paleo-Pacific Plate. The upwelling of asthenospheric mantle and local delamination of the lithosphere, which were induced by rollback of the subducting Paleo-Pacific Plate, extended from the Great Xing'an Range southeastward through the Songliao Basin to eastern Heilongjiang and Jilin provinces, giving rise to the southeastward migration of lithospheric extension and extension-related volcanism after ca. 140 Ma.

Reconnaissance Basement Geology and Tectonics of South Zealandia

AbstractWe report new U‐Pb zircon ages, geochemical and isotopic data for Mesozoic igneous rocks, and new seismic interpretations of mostly submerged South Zealandia (1.5 Mkm2). We use these data, along with existing geological and geophysical data sets, to refine the extent and nature of geological units. Our new 1:25 M geological map of South Zealandia provides a regional framework to investigate the rifting and breakup that formed Zealandia, Earth's most submerged continent. Samples of prerift (pre‐100 Ma) plutonic rocks can be matched with on‐land New Zealand igneous suites and indicate an east‐west strike for the subduction‐related 260 to 105‐Ma Median Batholith across the Campbell Plateau. The plutonic chronology of formerly contiguous plutonic rocks in West Antarctica reveals similar pulses and lulls to the Median Batholith. Contrary to previous interpretations, the Median Batholith does not coincide with the 1,600‐km‐long Campbell Magnetic Anomaly System. Instead we interpret the continental magnetic anomalies to represent a mainly mafic igneous unit, whose shape and extent is controlled by synrift structures related to Gondwana breakup. Correlatives of some of these unsampled igneous rocks may be exposed as circa 85 Ma alkalic volcanic rocks on the Chatham Islands. Extension directions varied by up to 65° from 100 to 80 Ma, and we suggest this allowed this large area to thin considerably before final rupture to form new oceanic crust. Synrift (90–80 Ma) structures cut the oroclinal bend in southern South Island and support a pre‐early Late Cretaceous age of orocline formation.

Late Triassic high‐Mg diorites and associated mafic dikes from the southern Zhangguangcai Range (NE China): Melting of juvenile arc crust and subarc mantle lithosphere

The Lesser Xing'an–Zhangguangcai Range is located in the eastern part of the Central Asian Orogenic Belt (CAOB). Early Mesozoic igneous rocks in this area can place constraints on the crustal growth mechanism in the Northeastern China. This paper reports a suite of Late Triassic high‐Mg diorites and mafic dikes from the southern Lesser Xing'an–Zhangguangcai Range. The mafic dikes have younger zircon U–Pb ages of 195 ± 2 Ma. They display intermediate SiO2 (50.58% to 58.20%) and high Na2O/K2O ratios of 1.90 to 3.20, low A/CNK ratios of 0.75 to 0.84, low (87Sr/86Sr)i ratios of 0.7045 to 0.7046, and high εNd(t) values of +3.1 to +3.5. Most zircons in the mafic dikes exhibit positive εHf(t) values from +8.6 to +16.0, with single‐stage Hf model ages ranging from 0.2 to 0.5 Ga in combination with their enrichment in LREE and LILEs and depletion in HFSEs. They are considered to be formed by melting of metasomatized subarc lithosphere mantle. The diorite and quartz diorite have older zircon U–Pb ages of 200 ± 3 and 202 ± 2 Ma, respectively. They have SiO2 = 54.45% to 61.18%, Al2O3 = 17.16% to 19.10%, TiO2 = 0.74% to 1.13%, and Mg# = 46.6 to 49.5, and they also display moderately depleted Sr–Nd (εNd(t) = +2.3 to +4.3) and zircon Lu–Hf (εHf(t) = +8.6 to +16.0) isotopic compositions. These features suggest that they were formed by melting of juvenile arc crust in active continental margin. Considering the occurrence of the contemporaneous and latter igneous rocks in the Zhangguangcai Range, we argue that the diorite, quartz diorite, and mafic dikes from the Lengshan area were also related to the westward subduction of Palaeo‐Pacific Plate (Izanagi Plate) beneath the Eurasian Plate. The strike‐like fault along the eastern margin of the Songneng–Zhuangguangcai Block may play an important role in the genesis of diorite melt.

Geochronology and geochemistry of Mesozoic igneous rocks of the Hunjiang basin, Jilin Province, NE China: Constraints on regional tectonic processes and lithospheric delamination of the eastern North China block

The Hunjiang basin in Jilin Province, China, is host to a NE-trending sequence of sedimentary rocks located along the northeastern margin of the North China block in the Paleoproterozoic Jiao-Liao-Ji belt. The North China block is the largest and oldest cratonic block in China, but unlike other Precambrian cratons, its eastern part has been decratonized by multiple Phanerozoic orogenic events. No comprehensive geochronology study has been conducted on the rocks of the Hunjiang basin, and both the mechanisms and timing of lithospheric delamination beneath the eastern part of the North China block are largely unconstrained. In the Hunjiang basin, Proterozoic to Paleozoic sedimentary rocks are intruded by Mesozoic granitoids and are overlain unconformably by sedimentary and volcaniclastic rocks. Twenty igneous rocks (granites, porphyritic rocks, and volcaniclastic rocks) from the basin were analyzed for whole-rock geochemistry and dated by U-Pb laser ablation-inductively coupled plasma-mass spectrometry of magmatic zircons. The samples collected are felsic to intermediate in composition and of calc-alkaline affinity, and the granitic and porphyritic rocks show an adakite-like geochemistry. With the exception of one Neoarchean granite dike with a 207Pb/206Pb age of 2524 ± 9 Ma that intrudes the basement along the northeastern margin of the basin, all igneous rocks dated in the present study have Mesozoic crystallization ages between 180 ± 1 Ma and 90 ± 2 Ma. The Precambrian inherited zircons show age peaks at ~2700 Ma, ~2650 Ma, and ~2500 Ma. The presence of inherited zircons with ages as old as 3387 ± 38 Ma (207Pb/206Pb ages) shows that Paleoarchean crust exists beneath the Proterozoic to Paleozoic sedimentary rocks of the basin. The geochronological results suggest that magmatic activity and associated formation of hydrothermal ore deposits in the Hunjiang basin were prevalent from the Middle Jurassic to the early Late Cretaceous. Furthermore, structural constraints on the cessation of major tilting in the basin are provided, as an Early Cretaceous (127 ± 1 Ma) flat-lying volcaniclastic rock unit unconformably overlies steeply dipping sedimentary rocks. The adakite-like geochemistry of most granitic and porphyritic rocks suggests partial melting of a delaminated eclogitic lower crust, whereas the higher MgO, Ni, and Cr content of the Early Cretaceous porphyritic rocks indicates reaction between adakitic melts and mantle peridotite. Many of the volcaniclastic rocks have more evolved, less adakitic signatures and show negative Eu anomalies, indicating these formed from more evolved melts at shallower crustal depths, accompanied by assimilation-fractional crystallization processes. The results of this study indicate that major delamination was completed in the Hunjiang basin by the Middle Jurassic, while delamination was ongoing into the Early Cretaceous in many other parts of eastern China.

Zircon U-Pb geochronology of crystal tuff on Lingshan Island and its geological implications for magmatism, stratigraphic age and geological events

Due to the unique location in the Ludong region, geochronological study of this area is essential for the understanding of the Cretaceous tectonic evolution of Eastern China. Sedimentary sequences interbedded with tuff layers unconformably overlay metamorphic rocks in the Sulu Orogen. This research presents a more reliable geochronological dataset of a tuff layer on Lingshan Island in Qingdao. A total of 103 valid age values from 216 zircon grains were obtained in three fresh tuff samples. Approximately 87% of these zircon ages are dated as the Early Cretaceous, and their peak ages shift from the Aptian stage to the Albian stage. The spatial-temporal relationship between the tuff and the Mesozoic igneous rocks of Eastern China indicate the impact of the Pacific Plate subduction beneath the Asian continent. Six Albian single detrital zircons have a weighted average age of 103.8 ± 1.4 Ma, with the youngest age (103.4 ± 1.4 Ma) constraining the maximum depositional age of the tuff layer. The age sequence of four sections on Lingshan Island is defined in this study: sections A and B belong to the Laiyang Group, and sections C and D are considered the Qingshan Group and were deposited in the Late Cretaceous. Two pre-Cretaceous zircon age peaks were also observed. These age peaks coincide with the magmatic and metamorphic ages preserved in the Sulu Orogen; thus, the Sulu Orogen is the provenance of the sedimentary rocks on Lingshan Island.

Rift-related magmatism on magma-poor margins: Structural and potential-field analyses of the Mesozoic Notre Dame Bay intrusions, Newfoundland, Canada and their link to North Atlantic Opening

The magma-poor Newfoundland margin formed following lithospheric stretching, rifting and breakup resulting in the separation of the Grand Banks from Iberia, and northern Newfoundland from Ireland. Mesozoic igneous rocks contemporaneous with rifting have been documented on- and offshore Newfoundland, with later offshore magmatism occurring post-breakup. The Notre Dame Bay Magmatic Province (NDBMP) is a Jurassic-Cretaceous, potentially rift-related occurrence of igneous rocks onshore Newfoundland that includes the gabbroic Budgell Harbour Stock (BHS) and marginal lamprophyre dykes, in addition to the nearby, potentially related, Dildo Pond Intrusion (DPI) and Leading Tickles Stock (LTS). Although the BHS is penetrated by wells, it is poorly exposed at surface, and the deeper structure remained unknown. Here, results of field-based mapping combined with inversion of Full Tensor Gradiometry (FTG) and aeromagnetic data covering the BHS are presented. Although the FTG data are of a higher resolution than the magnetic data, comparable results were obtained from these independent datasets. We found that the dykes form clusters at the ends of density and susceptibility anomalies, interpreted to be lobe-like magmatic conduits. Furthermore, potential-field analysis has revealed the irregular geometry and southward-dipping nature of the BHS, in addition to the presence of multiple, near surface anomalies that may correspond to dyke clusters or other igneous bodies. Structural analysis indicates that emplacement of the dykes was controlled by pre-existing geological structures and that they have probably been deformed post-intrusion, potentially via the reactivation of pre-rift faults. The potential-field investigations and observations of dyke margin lineations from field-based structural analysis indicate that multiple dyke sources and bodies similar to the BHS may also exist. Finally, this study demonstrates that magma-poor margins, such as the Newfoundland margin, may host significant volumes of extension-related magmatic rocks, challenging the concept of a distinction between magma-rich and magma-poor rifting.

Geochronology and petrogenesis of the granites in Malanyu Anticline in eastern North China Block

Widespread Mesozoic magmatism and intracontinental deformation is recorded in the North China Block (NCB), but few studies have considered the link between the emplacement of the granite and deformation. The Malanyu Anticline is a large-scale east-trending fold exposed in the eastern part of NCB and is intruded by many plutons. Whole rock geochemistry, LA-ICP-MS zircon U–Pb ages, Sr–Nd isotopic composition, as well as zircon Hf isotopic data for eight granitic bodies in the anticline have been studied in detail and are reported in this contribution. Fourteen zircon LA-ICP-MS U-Pb ages have been obtained ranging from ca. 218 to 126 Ma spanning the Triassic to Cretaceous, with a major population at ca. 220–160 Ma. This age range is different from rest of the Mesozoic igneous rocks in NCB, indicating the emplacement of the granites in the anticline was structurally controlled, although the regional tectonic setting during the Mesozoic played an important role in magmatism in the region. All of the granites in the Malanyu Anticline are enriched in light Rare Earth Elements relative to heavy Rare Earth Elements, and have similar trace elements patterns, with depletions in Ba, Nb, Sr, P and Ti, and enrichments in Rb, Th, U, K, La, Ce, Sm and Tb. The Triassic to Jurassic granites' two-stage depleted mantle Nd model (T2DM) ages are both Paleoproterozoic, with similar εNd(t) values and two-stage depleted mantle Nd model ages. Their in-situ zircon two-stage depleted mantle Hf model ages (tDM2) range from ca. 2861 to 1406 Ma. Integrated with the Sr-Nd-Hf data from previous studies, the Triassic Dushan Granite in the study area has high Sr and low Y assays derived from the partial melting of Archean TTGs in the lower crust with a minor contribution from a depleted mantle source. The emplacement was during a post-collisional setting following the closure of the Paleo-Asian Ocean along the Suolun Suture in the north. The emplacement of most of the Jurassic granites involved the superimposition of the initial subduction of the Paleo-Pacific Ocean plate beneath NCB, while the emplacement of the minor Jurassic and Cretaceous granites are probably related to the partial melting of the Precambrian crust due to widespread extensional tectonics at the northern margin of the NCB.

Late Jurassic to Early Cretaceous volcanism of Hong Kong: Insights from the Ping Chau Formation

The vast expanse of Mesozoic igneous rocks in Hong Kong contain important geological records of late Mesozoic magmatic events and tectonic processes from the coastal region of Southeast China. Of these, the Ping Chau Formation in the northwestern New Territories is the youngest known stratum. We perform a detailed study of the volcanic rocks of the Ping Chau Formation utilizing zircon U–Pb dating, with major and trace elements geochemistry. LA-ICP-MS zircon U–Pb data reveal Early Cretaceous age from two volcanic rock samples, with zircon crystallization from magmas at 140.3 ± 0.8 Ma and 139.3 ± 0.9 Ma, respectively. These rocks have high contents of total alkalis (Na2O + K2O = 5.58–9.45 wt.%), high-field-strength elements and light rare earth elements, conspicuous negative Eu anomalies, and depletions in Nb, Ta, Ti, Sr, Ba and P. Using this data, in combination with previous studies on the late Mesozoic volcanic belt in Southeast China, we propose that the volcanic rocks of the Ping Chau Formation probably originated from deep melting of the crust in a back-arc extensional setting induced by the subduction of the paleo-Pacific Plate. This formation represents the final stages of Early Cretaceous volcanic activity in Hong Kong, as associated with large-scale lithospheric extension, thinning and magmatism. Our results provide new information that can be used in evaluating the significance of Early Cretaceous volcanism and tectonics in Southeast China.

K-Ar・Rb-Sr年代からみた中生代火成作用

K-Ar・Rb-Sr年代からみた中生代火成作用

Geochemistry of Oligocene Huagang Formation clastic rocks, Xihu Sag, the East China Sea Shelf Basin: Provenance, source weathering, and tectonic setting

The East China Sea Shelf Basin (ECSSB) is an important tectonic unit of the western Pacific active continental margin. It is divided further into a series of sags and basement ridges. The Xihu Sag on the east margin of the ECSSB is an important hydrocarbon exploration area and contains significant petroleum resources. The Huagang Formation is the major reservoir in Xihu Sag, and its provenance has been the focus of debate at all times. The major and trace elements (including rare earth elements) compositions were analyzed for the Oligocene Huagang Formation sandstone and mudstone samples in the Xihu Sag. The provenance and tectonic setting of these clastic rocks are discussed. The Chemical Index of Alteration value and A–CN–K diagrams suggest low to moderate weathering in the source areas of the Huagang Formation. The mudstones have lower index of compositional variation values (<1), indicating that the mudstone samples are compositionally mature, whereas the index of compositional variation values for the sandstone samples (0.59 to 1.87) indicate that the sandstones are compositionally immature and mature. The major and trace element composition and ratios of the clastic rock samples indicate that mudstones are mainly from Late Mesozoic igneous rocks in SE China, which is dominated by the acidic volcanic rock. The provenance of sandstones is from two different directions, the Central Uplift Group of ECSSB and the Taiwan–Sinzi Fold Belt with more mafic volcanic rock. Based on the geochemical analysis of the clastic rock samples, the tectonic setting of Xihu Sag was Japanese‐type continental margin during the Huagang Formation deposition time (Oligocene).

하동-산청 회장암체 내 부존하는 하동 함 철-티탄 광체의 광화작용

하동-산청 회장암체는 영남육괴의 남서부에 선캠브리아기 지리산 편마암 복합체의 변성암류를 기반암으로 차노카이트(charnockite)와 함께 분포하며, 주변에는 중생대 화성암류가 관입 산출한다. 하동-산청 회장암체는 중생대 쥬라기의 섬장암을 경계로 북서쪽의 산청 회장암체와 남쪽의 하동 회장암체로 구분된다. 광체는 하동 회장암체 내에 남북방향으로 약 14 km의 연장을 보이며 단속적으로 산출되는 하동 함 철-티탄 암맥상 광체이다. 하동 함 철-티탄 암맥상 광체 내에는 함-철 산화광물인 자철석(magnetite) 및 티탄철석(ilmenite) 과 함께 함 티탄 광물들(금홍석(rutile) 과 티타나이트(titanite))과 소량의 황화광물들(자류철석, 황철석, 황동석, 섬아연석 등)이 수반하여 산출된다. 하동 함 철-티탄광체의 광화작용은 초기 자철석-티탄철석의 공생 산출로 시작되어 자철석-티탄철석 <TEX>${\rightarrow}$</TEX> 자철석-티탄철석-자류철석 <TEX>${\rightarrow}$</TEX> 티탄철석-자류철석-금홍석-티타니이트(<TEX>${\pm}$</TEX>황철석) <TEX>${\rightarrow}$</TEX> 황화광물의 공생관계를 보이며 진행되었다. 광체 내 공생관계와 및 열역학적 연구를 통하여 확인된 하동 함 철-티탄 광체의 초기 함 철-티탄 광화작용은 약 <TEX>$10^{-11.8}{\sim}10^{-17.2}$</TEX> atm의 산소 분압조건(<TEX>$700^{\circ}C$</TEX>)에서 <TEX>$Fe_3O_4-FeS$</TEX> 상평형을 이루는 황 분압조건 (약 <TEX>$10^0$</TEX> atm) 까지 황 분압의 증가에 의하여 진행되었으며, 그 후 황화광물의 산출은 산소 분압은 감소(<TEX>${\geq}10^{-20.2}$</TEX> atm)되면서 황 분압이 증가(<TEX>${\geq}10^0$</TEX> atm) 하는 환경에서 진행되었다. The Hadong-Sancheong Proterozoic anorthosite complex occurs in the southwestern region of the Ryongnam massif. The geology of the area mainly consists of metamorphic rocks of the Jirisan metamorphic complex as basement rocks, charnockite, and the Hadong-Sancheong anorthosite, which are intruded by the Mesozoic igneous rocks. Hadong-Sancheong anorthosite complex is divided into the Sancheong anorthosite and the Hadong anorthosite which occur at north-southern and south area of the Jurassic syenite, respectively. The Hadong Fe-Ti-bearing dike-like ore bodies developed intermittently in the Hadong anorthosite with north-south direction and extend about 14 km. The Hadong Fe-Ti-bearing ore bodies consist mainly of magnetite and ilmenite with rutile, titanite, and minor amounts of sulfides(pyrrhotite, pyrite, chalcopyrite and sphalerite). The Hadong Fe-Ti-bearing ore bodies show a paragenetic sequence of magnetite-ilmenite <TEX>${\rightarrow}$</TEX> magnetite-ilmenite-pyrrhotite <TEX>${\rightarrow}$</TEX> ilmenite-pyrrhotite-rutile-titanite(and/or pyrite) <TEX>${\rightarrow}$</TEX> sulfides. Equilibrium thermodynamic interpretation of the mineral paragenesis and assemblages indicate that early Fe-Ti-bearing ore mineralization in the ore bodies occurs at about <TEX>$700^{\circ}C$</TEX> which corresponds to oxygen fugacity of about <TEX>$10^{-11.8}{\sim}10^{-17.2}$</TEX> atm with the decrease tendency of sulfur fugacity to about <TEX>$10^0$</TEX> atm as equilibrium of <TEX>$Fe_3O_4-FeS$</TEX>. The change of ore mineral assemblages from Fe-Ti-bearing minerals to sulfides in late ore mineralization of the ore bodies indicates that oxygen fugacity would have slightly decreased to <TEX>${\geq}10^{-20.2}$</TEX> atm and increased sulfur fugacity to <TEX>${\geq}10^0$</TEX> atm.

Multidimensional classification of magma types for altered igneous rocks and application to their tectonomagmatic discrimination and igneous provenance of siliciclastic sediments

A new multidimensional scheme consistent with the International Union of Geological Sciences (IUGS) is proposed for the classification of igneous rocks in terms of four magma types: ultrabasic, basic, intermediate, and acid. Our procedure is based on an extensive database of major element composition of a total of 33,868 relatively fresh rock samples having a multinormal distribution (initial database with 37,215 samples). Multinormally distributed database in terms of log-ratios of samples was ascertained by a new computer program DOMuDaF, in which the discordancy test was applied at the 99.9% confidence level. Isometric log-ratio (ilr) transformation was used to provide overall percent correct classification of 88.7%, 75.8%, 88.0%, and 80.9% for ultrabasic, basic, intermediate, and acid rocks, respectively. Given the known mathematical and uncertainty propagation properties, this transformation could be adopted for routine applications. The incorrect classification was mainly for the “neighbour” magma types, e.g., basic for ultrabasic and vice versa. Some of these misclassifications do not have any effect on multidimensional tectonic discrimination. For an efficient application of this multidimensional scheme, a new computer program MagClaMSys_ilr (MagClaMSys—Magma Classification Major-element based System) was written, which is available for on-line processing on http://tlaloc.ier.unam.mx/index.html. This classification scheme was tested from newly compiled data for relatively fresh Neogene igneous rocks and was found to be consistent with the conventional IUGS procedure. The new scheme was successfully applied to inter-laboratory data for three geochemical reference materials (basalts JB-1 and JB-1a, and andesite JA-3) from Japan and showed that the inferred magma types are consistent with the rock name (basic for basalts JB-1 and JB-1a and intermediate for andesite JA-3). The scheme was also successfully applied to five case studies of older Archaean to Mesozoic igneous rocks. Similar or more reliable results were obtained from existing tectonomagmatic discrimination diagrams when used in conjunction with the new computer program as compared to the IUGS scheme. The application to three case studies of igneous provenance of sedimentary rocks was demonstrated as a novel approach. Finally, we show that the new scheme is more robust for post-emplacement compositional changes than the conventional IUGS procedure.

Constraining timing and tectonic implications of Neoproterozoic metamorphic event in the Cathaysia Block, South China

The Cathaysia Block of the South China Craton includes a Proterozoic basement that experienced a prolonged Precambrian crustal evolution but to date lacks evidence of Proterozoic metamorphic ages. At Lichuan and Jianning, in the Wuyi Domain of the eastern Cathaysia Block, Proterozoic rock units include migmatized paragneiss of the Wanyuan Group and minor amphibolite of the Tianjingping Formation, which are enveloped by schist of Mayuan Group, and all are intruded by Paleozoic and Mesozoic igneous rocks. Detrital zircon grains from the Wanyuan paragneiss display metamorphic rims that yield concordant weighted average 206Pb/238U ages of 860±6Ma and 435±5Ma, along with variably disconcordant ages with lower intercept ages of 442±41Ma. The zircon core ages range from 3015Ma to 851Ma, with three major age populations at 930–865Ma, 1850–1200Ma and 2650–2400Ma. Detrital zircon grains from Mayuan schist samples at Jianning generally lack core-rim structures and yield three main age populations at 860–736Ma, 1835–1775Ma and 2720–2500Ma. Metamorphic ages of ca. 860Ma and ca. 435Ma for the Wanyuan paragneiss along with the youngest detrital zircon constrain the depositional age of the protolith to ca. 865–860Ma, whereas the Mayuan Group is younger and probably deposited after ca. 736Ma. Characteristics of detrital zircon age populations along with regional geological data suggest accumulation of the Wanyuan Group in a convergent and/or collisional setting. Metamorphism and a possible subduction-collision process within the Cathaysia Block at around 860Ma suggest it was not a unified block in early Neoproterozoic. The growth of ca. 440Ma metamorphic rims is likely related to granitic magmatism, such as that exposed in the Lichuan region. The sparse evidence for early Neoproterozoic metamorphism likely reflects widespread overprinting by the Paleozoic tectonothermal event at around 440Ma.

Geochronology and geochemistry of late Paleozoic–early Mesozoic igneous rocks of the Erguna Massif, NE China: Implications for the early evolution of the Mongol–Okhotsk tectonic regime

We undertook geochemical and geochronological studies on late Paleozoic–early Mesozoic igneous rocks from the Erguna Massif with the aim of constraining the early evolution of the Mongol–Okhotsk tectonic regime. Zircon crystals from nine representative samples are euhedral–subhedral, display oscillatory growth zoning, and have Th/U values of 0.14–6.48, indicating a magmatic origin. U–Pb dating of zircon using SIMS and LA–ICP–MS indicates that these igneous rocks formed during the Late Devonian (∼365Ma), late Carboniferous (∼303Ma), late Permian (∼256Ma), and Early–Middle Triassic (246–238Ma). The Late Devonian rhyolites, together with coeval A-type granites, formed in an extensional environment related to the northwestwards subduction of the Heihe–Nenjiang oceanic plate. Their positive εHf(t) values (+8.4 to +14.4) and Hf two-stage model ages (TDM2=444–827Ma) indicate they were derived from a newly accreted continental crustal source. The late Carboniferous granodiorites are geochemically similar to adakites, and their εHf(t) values (+10.4 to +12.3) and Hf two-stage model ages (TDM2=500–607Ma) suggest they were sourced from thickened juvenile lower crustal material, this thickening may be related to the amalgamation of the Erguna–Xing’an and Songnen–Zhangguangcai Range massifs. Rocks of the late Permian to Middle Triassic suite comprise high-K calc-alkaline monzonites, quartz monzonites, granodiorites, and monzogranites. These rocks are relatively enriched in light rare earth elements and large ion lithophile elements, and depleted in heavy rare earth elements and high field strength elements. They were emplaced, together with coeval porphyry-type ore deposits, along an active continental margin where the Mongol–Okhotsk oceanic plate was subducting beneath the Erguna Massif.

Igneous rocks of the West Sakhalin Terrane of Sakhalin Island

It has been determined that the Rozhdestvenka Formation of the West Sakhalin Terrane composed of Late Mesozoic igneous rocks is a fragment of the accretionary prism of the Rebun–Kabato–Moneron–Samarga island-arc system. Volcanic eruptions, as well as destruction of the Rebun–Kabato–Moneron–Samarga island-arc and the East Sikhote-Alin volcano plutonic marginal continental belt, were the sources of pyroclastic and clastic material entering the sedimentary basin, where the Pobedinsk and Krasnoyarka suites of the West Sakhalin Terrane were formed.