Jurassic Volcanic Rocks Research Articles

Evaluation of the geochemical background of soil in a hyper-arid zone using a multivariate statistical methodology: The case of the city of Antofagasta in the Atacama Desert

This study investigates the geochemical background and factors influencing the variability of 19 environmentally relevant elements in the soils of Antofagasta, Chile, a region known for its extensive mining activities. Employing robust multivariate statistical techniques on a dataset of 94 soil samples, we identified four main factors explaining 70% of the total variance in elemental concentrations. These four factors reflect the influence of Jurassic volcanic rocks, intrusive rocks, marine sediments, and mafic to intermediate intrusive rocks. Cluster analysis revealed three distinct geochemical populations, each reflecting a unique combination of natural and anthropogenic influences. We established background concentrations for each element within these clusters using robust statistical methods. Geostatistical analysis, employing inverse distance weighted interpolation, produced factor distribution maps that, when integrated with geological data, provided insights into lithological and anthropogenic influences on soil geochemistry. Our findings highlight the complex interplay between natural geological processes, the region's unique arid climate, and anthropogenic activities in shaping the geochemical landscape of Antofagasta. This study contributes to the understanding of geochemical backgrounds in mining-intensive, arid regions and provides a methodological framework applicable to similar environments worldwide.

Spatiotemporal distribution and mechanism of Mesozoic magmatism in the Taiwan Strait and its adjacent areas: insights from seismic, geochemical, and geochronological data

ABSTRACT This paper presents a complete map of the Mesozoic volcanic-plutonic complexes in the Taiwan Strait and its adjacent areas in terms of seismic profiles, ages, geochemistry, isotopic systematics, and published data. Two periods of igneous activity in the Taiwan Strait can be distinguished by seismic profiles: Jurassic (200.4–148.2 Ma); and Latest Jurassic/Cretaceous to Maastrichtian (144–66 Ma), of which the Cretaceous is the best preserved and could possibly be the most widespread. Mesozoic magmatic activity with the NE-striking distribution was migrating southeastward (oceanward). LA-MC-ICPMS zircon U – Pb dating yields the age data of 109.5 ± 0.4 Ma for PTDH1 granodiorite and 92.6 ± 0.4 Ma for PTDH4 rhyolite in the Taiwan Strait, respectively. The 581.66-m-thick Upper Jurassic volcanic rocks of the Chuan2 well are the thickest Mesozoic igneous rocks in the Taiwan Strait. The petrogenetic discrimination diagram of the Cretaceous volcanic-plutonic complexes along the coast of South China and the Taiwan Strait generally is belonging to volcanic arc or intraplate type calc-alkaline rock series. The high potassium calc-alkaline rock series of the Cretaceous volcanic-plutonic complexes that is most widely distributed in the coastal areas of South China and the Taiwan Strait. They are controlled by the subduction of the Palaeo-Pacific domain.

Late Jurassic Tethyan igneous records in North Sumatra: Geochronological and geochemical constraints

The Jurassic igneous rocks on Sumatra Island are important for investigating the Mesotethyan evolution and regional correlation in Southeast Asia. This paper presents new laser ablation−multicollector−inductively coupled plasma−mass spectrometry zircon U-Pb-Hf geochronological and isotopic, whole-rock elemental, and Sr-Nd isotopic data for the newly identified Glebruk dolerite and andesite in North Sumatra. New zircon dating results suggest that these mafic−intermediate volcanic rocks, with ages of 150−146 Ma, were formed during the Late Jurassic. The Glebruk dolerite and andesite belong to the calc-alkaline series, are enriched in the light rare earth elements, and exhibit depletion in Nb, Ta, and Ti. Thus, they resemble arc-like volcanic rocks. They have low (87Sr/86Sr)i ratios of 0.7038−0.7048 and strong positive εNd(t) and εHf(t) values of +6.4 to +7.5 and +4.8 to +18.6, respectively. Their geochemical signatures suggest that these Late Jurassic volcanic rocks were derived from a depleted mantle wedge that was metasomatized by the slab-derived melts. The Glebruk volcanic rocks were formed in a continental arc setting in response to the Late Jurassic northward subduction of the Woyla Ocean beneath West Sumatra. Our results and regional geological observations suggest that the Mesotethyan subduction-related igneous rocks could extend from South Qiangtang through West Yunnan to North Sumatra. The Woyla Ocean could represent the southern extension of the Mesotethyan Ocean.

Metal sources and fluid pathways of karst-hosted Mississippi Valley-type Zn–Pb deposits in the fold-thrust belt: A case study of the Changdong deposit in the southeastern Himalayan-Tibetan orogen

The metal sources and fluid pathways for Mississippi Valley-type (MVT) Zn–Pb deposits in fold-thrust belts remain open topics of debate. To develop a more comprehensive understanding of these issues, this study presents the sulfide morphology, S and Pb isotopes, and trace element data of the paleokarst-hosted Changdong MVT Zn–Pb deposit in the “Sanjiang” fold-thrust belt. Bulk Pb isotopic ratios for galena were homogenous and indicated potential metal sources. These sources include Triassic to Early Jurassic volcanic rocks at the western margin of the Simao Basin, Cenozoic intrusive rocks at the eastern margin, and Jurassic clastic rocks. The spatial distribution of the in situ Pb isotopic ratios, combined with the spatial relationship between the faults and ore bodies, suggest that the tensile faults acted as vertical pathways for the fluid. The galena grains displayed oscillatory zones of As and Sb elements, indicating that alternating ore-forming fluids with varying chemical compositions entered the local reservoir. The δ34SV-CDT values of sulfides in substage I range from −19.3 to 29.6 ‰, and in substage II, they range from −21.2 to 52.6 ‰. The high fractionation degree (approximately 38.9 ‰) and the presence of microspherulitic morphology suggest that the reduced sulfur originated from a bacterial sulfate reduction. A GGIMFis geothermometer revealed that the precipitation temperatures of substage I sphalerite ranged from 56.68 to 211.58 °C and were higher than substage II. This indicated that the mixing of Zn–Pb-bearing fluid with reduced sulfur was the main controlling factor for sphalerite precipitation, and the temperature decrease represented another important factor. In the fold-thrust system, the complete process of Zn–Pb mineralization was established as follows: the fluid extracted metals from regional rocks and then migrated laterally over long distances. During the regional transition from compression to extension, metal-bearing fluid was injected into the deposit along the normal faults through seismic pumping. The fluid was then mixed with reduced sulfur to precipitate Zn–Pb sulfides. This study provides new insights into the metal sources and fluid pathways of MVT Zn–Pb deposits developed in a fold-thrust belt.

Petrogenesis of Early Mesozoic volcanic rocks in southeastern NE China: Geochemical and Sr–Nd–Pb–O isotopic evidences

This paper reports a study of whole-rock geochemical, Sr–Nd–Pb isotopic compositions and zircon UPb ages and O isotopic compositions of Late Triassic to Early Jurassic volcanic rocks of the Yanbian–Dongning area, northeastern China. The obtained data are used to constrain the petrogenesis of these rocks and to provide new insights into the timing of transition from the Paleo-Asian to the Paleo-Pacific tectonic regime. The studied Late Triassic to Early Jurassic volcanic rocks can be divided into two periods: Late Triassic–early Early Jurassic (212–198 Ma) and late Early Jurassic (ca. 181 Ma). The former rocks display high SiO2 and total alkalis contents, high Ga/Al ratios, depleted SrNd and highly radiogenic Pb isotopic signatures, and low zircon δ18O values. The former rocks have a genetic relationship with coeval intrusions, and their primitive melt was derived from juvenile crustal materials and underwent subsequent differentiation and assimilation of low-δ18O rocks. In contrast, the latter rocks have SiO2 contents of 49.00–63.30 wt%, are classified as calc-alkaline, and display arc-type trace-element features. Their geochemical and Sr–Nd–Pb–O isotopic compositions imply derivation from the partial melting of depleted mantle wedge metasomatized by oceanic slab-derived fluid and sediment melt. Combining these new results with published data, the Late Triassic–early Early Jurassic volcanic rocks are inferred to have formed in a rift-like extensional setting, whereas the late Early Jurassic volcanic rocks were generated in a subduction-related setting. The transition from the Paleo-Asian to the Paleo-Pacific tectonic regime occurred during the Early Jurassic.

Provenance Tracing and Age Analysis of Lead–Zinc Mineralization in Qiyimuchang, Inner Mongolia, NE China

The Qiyimuchang lead–zinc deposit is an important deposit in the Erguna Massif on the west slope of Daxinganling, for which the material source and age of mineralization remain unclear. The lead–zinc deposit in Qiyimuchang has been observed to occur in Jurassic volcanic strata as a vein-like orebody, and its strike is nearly perpendicular to that of the strata and the regional structures of the orogenic belt. The ore is mainly composed of sphalerite, galena, chalcopyrite, pyrite, and quartz, and hydrothermal alteration zones dominated by silicification, fluoritization, and pyritization are common within 1–5 m on both sides of the vein. The metasomatic lead–zinc mineralization is characterized by a massive vein structure. The mineralization process can be divided into three stages, pre-ore (mainly quartz, arsenopyrite and a small amount of pyrite), ore-formation (pyrite, chalcopyrite, sphalerite, galena, fluorite, and a small amount of tetrahedrite), and post-ore (quartz-calcite veinlets with a small amount of pyrite). Element and isotope geochemical studies show that the trace element compositions of the wall rocks (andesite, tuff, etc.) of Jurassic volcanic rocks in the ore bodies and surrounding rocks reflect affinity or similarity between them. The S isotopic composition of sulfide minerals in the metallogenic stage (δ 34SV-CDT = 1.6‰~4.8‰) indicate the sulfur isotopic composition of the magma. The Pb isotopic compositions of sulfide minerals (38.5–38.39, 15.55–15.62, and 18.33–18.41 for 208Pb/204Pb, 207Pb/204Pb, and 206Pb/204Pb, respectively) in the metallogenic stage are relatively concentrated and close to the average Pb isotopic compositions of the mantle and average Pb isotopic composition of an orogenic belt, indicating that the ore-forming metals are likely to be a mixed source of crust and mantle. A new zircon U-Pb age of 150.8 ± 1.3 Ma (MSWD = 0.74) was obtained from andesitic tuff. Collectively considering the regional geology and ore deposit geological analysis, the lead–zinc mineralization in Qiyimuchang possibly occurred during the early Cretaceous. The ore-forming processes belong to Cretaceous magmatic-hydrothermal activity in extensional tectonic setting.

The Au[sbnd]Ag (Zn, Pb, Mo, Cu) Sulfuro Vein, La Paloma district, Deseado Massif, Argentina: Geochemical characterization and new insights into the 4D evolution of ore shoots

Unravelling the 3D architecture of ore shoots and its evolution through time, thereby moving towards a 4D understanding of mineralization processes, requires an interdisciplinary approach based on the capability of carrying out extensive trenching and drilling as well as effectively integrating structural and geochemical analyses. Such conditions are offered in La Paloma district of the Deseado Massif, Argentina. Here, eight epithermal AuAg veins are hosted in Middle Jurassic volcanic rocks (Bajo Pobre Formation). The AuAg (Zn, Pb, Mo and Cu) Sulfuro Vein, representing the main ore body in the district, is a 750 m long, N to NW striking structure extending 230 m at depth. The geometry and distribution of ore shoots within the Sulfuro Vein are controlled by: (i) lithological and structural features, (ii) metal concentration, (iii) temperature of the fluids at the time of ore deposition, and (iv) remobilization processes. The highest values of Au, Ag, Cu, Mo, Pb, Zn and Sb are concentrated at depths between 50 and 100 m.a.s.l., while high Mo values occur also at greater depths in the southern segment of the vein. Molybdenum distribution in shallower sectors of the vein is controlled by its remobilization by later infill stages. The Au, Ag and Cu ore shoots are widely distributed in the southern and central segments of the vein, as are the areas of greater vein thickness. These ore shoots exhibit a sub-horizontal geometry consistent with dominant extensional faulting during mineralization. In the northern segment of the vein, the Au, Ag and Cu ore shoots are discontinuous and small, and show gentle to moderate plunges probably associated with variable fault kinematics and depletion of the fluid in these metals. Ore-shoots of Mo, Pb and Zn display high values along the longitudinal section and sub-horizontal geometry in the central and northern sectors of the vein, with high-grade Mo ore shoots decreasing to the north. The fact that Pb and Zn high grades extend up to the tip of the northern vein segment suggests that these metals continued to precipitate at lower temperatures, favoured by the permeability of the volcaniclastic units. All of the ore shoots exhibit steeper plunges towards the southern termination of the vein. Here, upward fluid flow may have been enhanced by the dilation associated with oblique-slip along the N-S striking segment of the steeply dipping Sulfuro Vein. The geochemical distribution of metals shows a slight vertical zonation and a distinct lateral zonation, which suggest hydrothermal fluids flowed northward from deeper zones in the southern sector of the vein.

Migration of Middle-Late Jurassic volcanism across the northern North China Craton in response to subduction of Paleo-Pacific Plate

The primary driver of magmatism, crustal deformation and metallogeny of the North China Craton in the Jurassic remains actively debated, either attributed to the subduction of the Paleo-Pacific (i.e., Izanagi) plate in the east or to the closure of the Mongol-Okhotsk Ocean in the north. This issue is addressed here by examining temporal and spatial variation of the contemporary volcanic rocks (i.e., Tiaojishan Formation) of the Yanshan belt in the northern part of the craton. These rocks are mainly calc-alkaline and intermediate-felsic in composition and show a highly coherent geochemical composition and unradiogenic zircon Hf isotope, indicating that they have similar origins and formed in a subduction-related setting. The eruption age of the Jurassic volcanic rocks decreases westward (i.e., inland-ward) from Western Liaoning (166–153 Ma), through Pingquan-Chengde-Luanping (162–153 Ma) and Jingxi-Xuanhua (158–149 Ma), to Yuxian (157–142 Ma). This temporal and spatial variation is consistent with the motion of the subducted Paleo-Pacific plate over that time. In addition to the westward migration, the earliest volcanic rocks in the Yanshan belt formed earlier than those in the adjacent southern Great Xing'an Range in the north. These observations suggest that the southward subduction and closure of the Mongol-Okhotsk plate played a limited role in generation of these volcanic rocks. Two episodes of Middle-Late Jurassic volcanism (166–153 Ma and 150–139 Ma) are observed in the northern Great Xing'an Range. They are interpreted as results of closure of the Mongol-Okhotsk Ocean and subduction of the Paleo-Pacific plate, respectively. Temporal-spatial variation of volcanic activity in the northern North China Craton and adjacent region demonstrates that the Paleo-Pacific subduction was the principal cause of the evolution of Eastern China during the late Mesozoic, whereas the Mongol-Okhotsk tectonic regime is largely confined to the northern Great Xing'an Range and the area north to it.

Discovery of the Mid–Late Jurassic volcanic rocks (163.4 Ma) in Nanhuangcheng Island, Shandong Province, China

Discovery of the Mid–Late Jurassic volcanic rocks (163.4 Ma) in Nanhuangcheng Island, Shandong Province, China

Metallogeny of the Late Jurassic Qiucun epithermal gold deposit in southeastern China: Constraints from geochronology, fluid inclusions, and H-O-C-Pb isotopes

On the basis of zircon and apatite U-Pb dating, H-O-C-Pb isotopes, and fluid inclusion data, a metallogenic model is established for the Qiucun gold deposit (gold resource > 10 t, average grade: 4.9 g/t) in the Dehua goldfield in the Mesozoic Coastal Volcanic Belt (southeastern China). Three hydrothermal stages can be distinguished within the Late Jurassic volcanic host rocks: Stage I, dominated by quartz-pyrite, Stage II with a quartz-sulfide assemblage, and Stage III reflected by quartz-calcite. The ore-hosting volcanic rocks were formed at 170.2 ± 0.9 Ma. Distinctly younger U-Pb ages of 162.5 ± 1.0 Ma and 160.5 ± 6.0 Ma obtained on syn-ore hydrothermal zircon and apatite, respectively, clearly indicate that gold mineralization postdates the volcanic host rocks. Fluid inclusion data reveal that the ores were precipitated from low-temperature (275 to 170 °C), low-pressure (<55 bar), and low-salinity (<5 wt% NaCl equiv.) solutions, with a systematic decrease in temperature and pressure from stages I to III. The δ34SV-CDT ratios for gold ores vary between −3.2 and 0.6 ‰. Sulfide separates yielded 206Pb/204Pb ratios of 18.368 to 18.504, 207Pb/204Pb ratios of 15.685 to 15.718, and 208Pb/204Pb ratios of 38.792 to 39.063. These data suggest S and Pb derivation from the host volcanic rocks and perhaps deeper-seated crustal materials. Calcite from Stage III has δ13CV-PDB values of −2.9 to −2.7 ‰ and δ18OV-SMOW values of 8.2 to 10.8 ‰. Calculated δ18Ofluid (−7.6 to −2.7‰) and δDfluid (−119 to −49‰) values document a largely meteoric nature of the mineralizing fluid. All data together point at a Late Jurassic low-sulfidation epithermal gold mineralization with metal remobilization from host volcanic rocks and basement.

Источник трахидацит-риолитовых галек среднеюрских конгломератов на северо-западном берегу Байкала: сопоставление галек с породами магматических комплексов верхнего палеозоя и мезозоя Забайкалья

From paleontological data, conglomerates exposed on the northwestern shore of Lake Baikal, in the Bolshiye Koty and Listvyanka areas, belongs to the upper part of the section of the southeastern part of the Irkutsk coal-bearing basin (Angara-Koty depression) and is considered as part of the lower subformation of the Koty Formation dated back to the upper part of the Aalenian Stage of the Middle Jurassic (174–170 Ma). To identify a source of clastic material of the conglomerates, petrogenic oxides and trace elements of pebbles of volcanic rocks are determined and thin sections are examined. Defined are five groups of trachydacite-rhyolite pebbles of plumasite composition in the former outcrop and a group similar in composition to the main Bolshiye Koty group in the second one. The difference between the pebbles of conglomerates from the upper part of the section in the Angara-Koty depression and those of alkaline effusives of acidic and intermediate composition from the lower and middle parts of the section is indicated. It is assumed that the composition of the pebbles of the Upper Koty subformation is similar to that of the Middle Jurassic volcanic and subvolcanic rocks from Transbaikalia. Their origin from three sources is possible: 1) from nearest one – an unknown Middle Jurassic volcano buried under Cenozoic sedimentary stratum of the South Baikal basin, in the suture zone of the Siberian paleocontinent, 2) from the middle-remote one (distance over 250–500 km) – Middle Jurassic volcano-plutonic complexes of the Dzhida-Vitim fault zone, and 3) from the more remote one – the Mongolian-Okhotsk fault zone.

Zircon U–Pb ages in El Chaltén Jurassic rhyolites (Chon Aike Silicic LIP, Argentina) reveal xenocrystic remnants of an unexposed lower basement of the Paleo-Pacific Gondwana margin

We investigate the Jurassic volcanic rocks of the El Quemado Complex in the El Chaltén region (Argentina), which correlate with the youngest eruptive event of the Chon Aike Silicic LIP in southern Patagonia. New zircon crystallization ages of the various silicic units were obtained using U–Pb geochronology measured by in-situ Laser Ablation Inductively Coupled Plasma Mass Spectrometry. The average ages for 12 samples range from 147 to 152 Ma. Xenocrystic zircon cores of Jurassic igneous zircons are characterized by complex cathodoluminescent textures wherein growth surfaces are truncated by dissolution surfaces that are in turn overgrown by euhedral growth zones that exhibit oscillatory magmatic zoning. The cores have a wide range of ages, from 185 Ma to 1.3 Ga. We combine our analyzed xenocryst zircon ages preserved in the El Quemado Complex rhyolites with a compilation of published xenocryst zircon ages documented from Cenozoic to Paleozoic volcanic and plutonic rocks throughout southern Patagonia. The combined datasets suggests three prominent geologic features not currently exposed on the present day surface: (1) widespread abundant Triassic through Permian (200–300 Ma) xenocrystic ages throughout the present day Andean region, which connect to current exposures in Northern Patagonian and that of the Antarctic Peninsula, marking the Paleo-Pacific Gondwanan margin, (2) the existence of widespread Early-Middle Ordovician to Cambrian ages throughout southern Patagonia, and (3) the presence of an old (∼1.0 Ga) crust below that of the Deseado Massif and Tierra del Fuego, which likely extends beneath the present-day Andes.

In situ mineral chemistry of chlorite in Donghua area, Dehua‐Youxi‐Yongtai ore district, Fujian Province, south‐east China: Elemental characteristics and their implications for exploration

As an ordinary alteration mineral in the process of fluid–rock interaction, chlorite (most especially its chemical composition) has the potential to become an effective tool to reveal the physicochemical conditions during alteration and help exploration. However, its elemental characteristics during the chloritization process are yet to be clearly understood. The Dehua‐Youxi‐Yongtai (DYY) ore district in south‐east China is one of the potential areas of gold and polymetallic deposits. Lying in the north‐west section of the DYY ore district, the Donghua area is notable for its remarkable metallogenic potential, and porphyry‐epithermal systems might have developed in the area. This study focuses on the elemental characteristics of chlorite in the Donghua area as footprints of the alteration process and mineralization vectoring. According to the geological features and occurrences, the chlorite from Donghua can be divided into two generations: (a) Chlorite I is closely related to chloritization developed in intrusive and volcanic rocks (Permian quartz monzobiorite and Jurassic volcanic rocks of Changlin Formation), and (b) chlorite II is accompanied by superimposed hydrothermal overprinting. The alteration process suggested by overprinting chlorite II can be regarded as almost coeval with the ca. 154–153 Ma magmatic event, and chlorite II is later than chlorite I generation. According to the geothermometry of the chlorite in Donghua, chlorite I might have crystallized on the temperature of 180–240°C with a peak of ~200°C, and the overprinting chlorite II might have experienced two episodes of hydrothermal/epithermal fluid pulsing. The mineral geochemistry of the trace elements in the two generations of the chlorite in Donghua shows different characteristics. The replacement of Mg2+ by Fe2+ plays an important role for ionic substitution in the octahedral position, especially for chlorite II. The other occasion might be Mg and Fe jointly entering the octahedral position as well as Mg‐ and Fe‐AlVI substitution mechanisms. The enrichment of Mg especially in chlorite II suggests low‐grade oxidation and acid conditions, which might be beneficial for the transportation of metallogenic substances. The obvious differences of Co and Ni are remarkable aspects of the trace elements of chlorite in Donghua, resulted by the ion substitution and function of different octahedral site preference energy. The mineral chemistry of chlorite II from Donghua can be helpful for targeting and exploration vectoring in the DYY ore district.

The volcanism of the Jurassic Chon Aike Silicic LIP influenced by Paleozoic inherited crustal structures in the northeastern Deseado Massif, Patagonia

An area of ∼40 km2 located in the northeastern sector of the Deseado Massif is detailed studied, in which Paleozoic metamorphic and Jurassic volcanic rocks crop out. Two genetically linked and compositionally different Jurassic volcanic events of the Chon Aike Silicic LIP are documented by geological mapping and lithofacies and petrographic analyses. The first event was only represented by andesitic lava flows assignable to the intermediate portion of the Bahía Laura Volcanic Complex (Bajo Pobre Formation). The second was represented by the acidic rocks of the Bahía Laura Volcanic Complex (Chon Aike and La Matilde formations) and it was constituted by low-grade and moderate grade ignimbrites, pyroclastic fall deposits (ash tuffs) and several lava bodies (rhyolitic lava domes and lava flows). Firstly, the effusive eruptions dominated during the intermediate magmatic eruptions, whereas the explosives mechanisms dominated during the first stage of the acidic magma extrusion, to end with effusive eruptions of previously degassed magma. According to the morphology of the NNW-elongated lava domes (feeder conduits plugs) and internal measurements carried out on these rhyolitic bodies, fissure conduits are interpreted, which were directly controlled by the Paleozoic inherited structures during the Jurassic extensional tectonic reactivation ascribed to the initial stages of the Gondwana breakup.

Age and Geochemistry of Late Jurassic Mafic Volcanic Rocks in the Northwestern Erguna Block, Northeast China

The northwestern Erguna Block, where a wide range of volcanic rocks are present, provides one of the foremost locations to investigate Mesozoic Paleo-Pacific and Mongol-Okhotsk subduction. The identification and study of Late Jurassic mafic volcanic rocks in the Badaguan area of northwestern Erguna is of particular significance for the investigation of volcanic magma sources and their compositional evolution. Detailed petrological, geochemical, and zircon U-Pb dating suggests that the Late Jurassic mafic volcanic rocks formed at 157–161 Ma. Furthermore, the geochemical signatures of these mafic volcanic rocks indicate that they are calc-alkaline or transitional series with weak peraluminous characteristics. The rocks have a strong MgO, Al2O3, and total alkali content, and a SiO2 content of 53.55–63.68 wt %; they are enriched in Rb, Th, U, K, and light rare-earth elements (LREE), and depleted in high-field-strength elements (HFSE), similar to igneous rocks in subduction zones. These characteristics indicate that the Late Jurassic mafic volcanic rocks in the Badaguan area may be derived from the partial melting of the lithospheric mantle as it was metasomatized by subduction-related fluid and the possible incorporation of some subducting sediments. Subsequently, the fractional crystallization of Fe and Ti oxides occurred during magmatic evolution. Combined with the regional geological data, it is inferred that the studied mafic volcanic rocks were formed by lithospheric extension after the closure of the Mongol-Okhotsk Ocean.

USING HIGH-RESOLUTION AMMONITE BIOCHRONOSTRATIGRAPHY TO DATE VOLCANOGENIC DEPOSITS PRESERVED IN MIDDLE JURASSIC CARBONATE PLATFORMS SUCCESSIONS OF THE WESTERNMOST TETHYS (SOUTHEASTERN IBERIAN RANGE, SPAIN)

The Middle Jurassic successions that are currently exposed in eastern Spain (southeastern Iberian Range) were deposited in a system of shallow carbonate platforms, constituting the western margin of the Alpine Ocean, and display a series of successively interbedded volcaniclastic rocks and minor lava flows. The large-scale mapping reveals that Middle Jurassic volcanic rocks crop out along NW‒SE pathways, highlighting the importance of the role played by tectonics in controlling the spatial distribution of volcanic outcrops. The progressive dismemberment of Pangea impelled a geographic provinciality increase and, thus, might have contributed to a growing speciation rate of marine invertebrate organisms, notably ammonites. The present study provides new data constraining in detail the age of the Middle Jurassic volcanic deposits. The biochronostratigraphic calibrations of the sedimentary host rocks encasing the volcanics have been obtained from the study of the fossil content (mainly ammonites) from16 well-exposed stratigraphic sections. The study of the fossils from these sections has enabled the identification of three volcanic episodes during the Murchisonae Zone (Aalenian), the Concavum‒Discites zonal boundary (Aalenian-Bajocian), and the late Laeviuscula Zone or Laeviuscula‒Propinquans zonal boundary (Bajocian). Precise age constraints for volcanic accumulations can be a significant contribution in reconstructing the geodynamic history of the Iberian Basin.

North China block underwent simultaneous true polar wander and tectonic convergence in late Jurassic: New paleomagnetic constraints

Jurassic witnessed two synchronous events on the Earth, named true polar wander (TPW) and closure of the Mongol–Okhotsk ocean (MOO) between Siberia and eastern Asian blocks. The paleogeographic change of the north China block (NCB) should thus include both TPW and plate tectonic components in Jurassic. We performed a new combined paleomagnetic and U–Pb geochronological study for Middle and Late Jurassic volcanic rocks from the northern NCB and obtained three new well-dated paleopoles: 82.7°N, 282.6°E, A95=3.2° (ca. 160 Ma); 84.1°N, 228.9°E, A95=6.6° (ca. 165 Ma); and 89.3°N, 216.8°E, dp/dm = 5.4°/7.1° (ca. 170 Ma). A positive untilting test indicates a pre-folding origin interpretation for the ca. 160 Ma pole. Quality of these poles is enforced by three positive conglomerate tests, the presence of magnetic reversals and secular variation analyses. Comparison of the apparent polar wander paths of Eurasia and the NCB suggests that the NCB accreted to Eurasia around 140 Ma. The NCB and its then connected blocks did not undergo any significant southward paleogeographic shift during Jurassic. In context of TPW, we estimate a net northward plate moving velocity at 0.50–0.75°/myr for the NCB in Jurassic. The long-term accumulation of MOO subducted slabs at the high paleolatitudes may have critically contributed to, or perhaps led to, the Jurassic TPW.

Geology and epithermal silver-gold vein mineralization of the San José mining district, Santa Cruz province, Argentina

The San José mining district in the northwestern Deseado Massif of southern Argentina produces silver and gold from intermediate sulfidation style epithermal veins.The district geology is characterized by a subhorizontal sequence of Jurassic volcanic rocks, dominated by volcanogenic agglomerates and lava flows of andesitic to basaltic andesitic composition of the Bajo Pobre Fm, that are locally overlain by felsic ignimbrites and air fall tuffs of the Chon Aike and La Matilde Fms. Several small hornblende-porphyritic subvolcanic stocks, as well as polymictic and matrix-supported breccias cross-cut the andesite sequence. Geochronology indicates that Jurassic volcanism was active in the district between 151.3 ± 0.7 and 144.7 ± 0.1 Ma, and mineralization formed from 151 to 141 Ma. More than 50% of the San José district is covered by post-Jurassic deposits including Cretaceous sedimentary and volcano-sedimentary rocks, Tertiary plateau basalts and modern periglacial gravel deposits.The Huevos Verdes-Frea vein cluster is the economic heart of the San José district and source of its production with more than 20 km of northwest-striking veins hosted by andesite lava of the Bajo Pobre Fm.West-northwest-striking veins (~N280° and ~N320°) display dextral strike-slip motion and contain ore shoots over strike lengths of up to 1200 m (Kospi, Odin, Frea, Ayelen). By contrast, north-northwest striking veins (>N320°) (Huevos Verdes) show sinistral strike-slip motion and host more discontinuous ore shoots, with strike length up to ~400 m separated by transverse syngenetic faults. Vein mineralization is characterized by three paragenetic stages. Pre-ore barren quartz and chalcedony is followed by the ore stage, consisting of mottled and crustiform quartz-sulfides and late sulfide-rich veinlets. Sulfide paragenesis shows repetitive cycles starting with pyrite, sphalerite, galena, chalcopyrite, minor bornite, and covellite, followed by precious metal-dominated pulses with electrum, acanthite/argentite, and rare silver sulfosalts. Gangue mineralogy is mainly characterized by quartz intergrown with minor adularia, illite, illite/smectite, and preferentially at deeper levels Fe-chlorite (±epidote), indicating precipitation from near-neutral pH hydrothermal fluids at temperatures of >220 °C. Kaolinite gangue accompanies illite/smectite with increasing presence towards higher levels and in later stages suggest fluid mixing with steam-heated fluids resulting in decreased fluid temperatures and pH towards shallower levels and over time.Other mineralized sectors of the district lack economic significance. The El Pluma-La Sorpresa vein cluster comprises narrow quartz veins and silicic ribs hosted by andesitic volcanic rocks of the Bajo Pobre Fm. with illite, illite/smectite and kaolinite alteration. Cerro Saavedra is an alteration system in the vent area of a partially eroded andesitic volcanic edifice with heterolithic explosion breccias where proximal residual quartz with vuggy texture is surrounded by a halo of quartz-alunite and distal illite alteration. At Saavedra Oeste a swarm of northwest-striking, short vein and breccia lenses is hosted by ignimbrites of the Chon Aike Fm. Hydrothermal alteration ranges from proximal silicification grading out to illite, illite/smectite and minor kaolinite.

Transition from a passive to active continental margin setting for the NE Asian continental margin during the Mesozoic: Insights from the sedimentary formations and paleogeography of the eastern Jiamusi Massif, NE China

Abstract The Mesozoic tectonic evolution of the NE Asian continental margin has received much attention in recent years. However, previous studies focused mainly on the petrogenesis of igneous rocks and their relationship with Mesozoic tectonics, and there have been few studies of the Mesozoic sedimentary formations of the NE Asian continental margin. We combined zircon U-Pb ages with Hf isotopic and biostratigraphic data to reconstruct the Mesozoic paleogeography of the NE Asian continental margin. The results indicate that Mesozoic strata of the eastern Jiamusi Massif, NE China, include the Upper Triassic Nanshuangyashan Formation (Norian), Lower Jurassic volcanic rocks, and Lower Cretaceous Longzhaogou Group. The Upper Triassic Nanshuangyashan Formation consists of a suite of alternating marine and terrestrial sedimentary rocks with abundant fossils that formed in a passive continental margin setting. The Lower Jurassic strata comprise a suite of calc-alkaline volcanic rocks that include basaltic andesites, andesites, and rhyolites that formed in an active continental margin setting related to initial subduction of the Paleo-Pacific Plate beneath Eurasia. The Lower Cretaceous Longzhaogou Group belong to alternating marine and terrestrial sedimentary formations that formed in an active continental margin setting related to subduction of the Paleo-Pacific Plate. Here, we integrate these data to reconstruct the Mesozoic tectonic history of the NE Asian continental margin, which comprises a Late Triassic passive continental margin, the initiation of subduction of the Paleo-Pacific Plate in the Early Jurassic, and westward subduction and rollback of the Paleo-Pacific Plate in the Early Cretaceous.

Chapter 2.2b Palmer Land and Graham Land volcanic groups (Antarctic Peninsula): petrology

Abstract Large-volume rhyolitic volcanism along the proto-Pacific margin of Gondwana consists of three major episodes of magmatism or ‘flare-ups’. The initial episode (V1) overlaps with the Karoo–Ferrar large igneous provinces at c. 183 Ma. A second (V2) episode was erupted in the interval 171–167 Ma, and a third episode (V3) was emplaced in the interval 157–153 Ma. The magmatic events of the V1 and V2 episodes of the Antarctic Peninsula are reviewed here describing major and trace elements, and isotopic (Sr, Nd, O) data from rhyolitic volcanic rocks and more minor basaltic magmatism. An isotopically uniform intermediate magma developed as a result of anatexis of hydrous mafic lower crust, which can be linked to earlier, arc-related underplating. The subsequent lower-crust partial melts mixed with fractionated mafic underplate, followed by mid-crust storage and homogenization. Early Jurassic (V1) volcanic rocks of the southern Antarctic Peninsula are derived from the isotopically uniform magma, but they have mixed with melts of upper-crustal paragneiss in high-level magma chambers. The V2 rhyolites from the northern Antarctic Peninsula are the result of assimilation and fractional crystallization of the isotopically uniform magma. This process took place in upper-crust magma reservoirs involving crustal assimilants with an isotopic composition akin to that of the magma. A continental margin-arc setting was critical in allowing the development of an hydrous, fusible lower crust. Lower-crustal anatexis was in response to mafic underplating associated with the mantle plume thought to be responsible for the contemporaneous Karoo magmatic province and rifting associated with the initial break-up of Gondwana.