Jurassic Granitic Rocks Research Articles

Petrogenesis of Jurassic granitic rocks in South China Block: Implications for events related to subduction of Paleo-Pacific plate

Abstract Frequent magmatic intrusion and eruption during the early Yanshanian in South China formed a unique and diverse igneous rock assemblage, mainly comprising granite, rhyolite, and some basic rocks. Nevertheless, the tectonic regime responsible for the formation of these granitic rocks remains controversial. The comprehensive available data suggest that the Jurassic granitic rocks formed during the period from 180 to 145 Ma, with an age peak at ca. 160 Ma. Jurassic rocks are predominantly concentrated in Wuyi, southern Hunan, northeast Guangdong, and southern Jiangxi, followed by the eastern Guangxi-western Guangdong areas, mainly including I-, S-, and A-type granites, and a few I–S transformation-type granites. These granitic rocks invariably exhibit enrichment in Rb, Th, U, and Pb, while Ba, Nb, Sr, P, and Ti are depleted, and variable Eu/Eu* ratios. The whole-rock Sr–Nd isotopic and zircon Hf isotopic compositions show that the Jurassic granitic rocks are mainly recirculation products of Paleo-Mesoproterozoic rocks, as well as the mantle-derived magma, which played a major role in the formation process. Among them, the granites in eastern Guangxi-western Guangdong and Northeast Guangdong areas display higher ε Nd(t) and ε Hf(t) values, suggesting a relatively larger contribution from mantle-derived materials. Based on the comprehensive regional geological data, we interpret that these Jurassic granitic rocks as a product of lithosphere extensional-thinning, tectonic-magmatic interaction formed during the process of flat-slab subduction.

Tectonic transition from Paleo- to Neo-Tethyan Ocean in Tangjia-Sumdo area, Southern Tibet: Constraints from Early Jurassic magmatism

The history Paleo-Tethyan Ocean basin in Tangjia-Sumdo area of Lhasa block has not been investigated in detail, particularly the tectonic evolution after its closure. To better understand this issue, we investigated U-Pb zircon chronology, zircon Hf isotopes, and whole rock geochemistry of Early Jurassic granitic rocks in Tangjia-Sumdo area on Lhasa Block. Three types of samples were obtained in this study, diorites (199.8 ± 1.1 Ma), monzogranites (194.3 ± 1.1 Ma and 200.5 ± 1.2 Ma), and granites (184.5 ± 2.1 Ma, 185.5 ± 4.0 Ma and 185.6 ± 2.1 Ma), which can be divided into two stages according to their eras. The whole rock geochemical characteristics show that all the samples are I-type granitoids. The geochemical analyses show that the first stage of magmatism was derived from partial melting of crustal material mixed with mantle material, and the second stage of magmatism was derived from partial melting of the crust. Based on the zircon Hf isotopic characteristics (the calculated εHf(t) values of the diorite are − 16.7 to − 7.7, monzogranites are + 1.8 to + 5.2, granites are + 1.6 to + 9.9), we conclude that diorites were derived from the ancient continental crust, and the other two groups of samples were derived from juvenile crust. Based on a comparison with previous research results and regional tectonic development, we suggest that the first stage of magmatism was the result of slab break-off of subducting Tangjia-Sumdo Paleo-Tethyan oceanic crust. The second stage of magmatism was the result of northward subduction of Neo-Tethyan Ocean, and it may represent tectonic transition from Paleo- to Neo- Tethys Ocean in Early Jurassic in Lhasa Block.

Early Jurassic granodiorite‐monzogranite‐granite suite in the Zhangguangcai Range, NE China: Implications for melting of newly accreted arc crust

High‐K calc‐alkaline granitoids play a key role in understanding the formation and evolution of the continental crust. Herein, we present new laser ablation inductively coupled plasma mass spectrometry zircon U–Pb and Lu–Hf isotopic, whole‐rock geochemical and Sr–Nd isotopic data of early Jurassic high‐K calc‐alkaline granitoids in the Zhanggunagcai Range, NE China. In situ zircon U–Pb dating shows that the granodiorite‐monzogranite‐granite (GMG) suite was emplaced at ca. 192–187 Ma. They are high‐K calc‐alkaline and weakly peraluminous, similar to the geochemical characteristics of I‐type granites. The GMG suite has lower bulk‐rock initial 87Sr/86Sr ratios of 0.7037–0.7042, slightly positive εNd(t) values of +1.10 to +1.87 and highly positive zircon εHf(t) values of +5.3 to +10.9. This result suggests a major contribution of juvenile crust to the magma source. The tetrad rare earth element patterns and highly negative Eu anomalies of samples with high SiO2 contents indicate that fractional crystallization plays an important role in the genesis. Together with data in the literature, we infer that the early Jurassic granitic rocks have been formed by partial melting of juvenile arc crust associated with the westward subduction of the Paleo‐Pacific Plate.

Tracking and Evaluating the Concentrations of Natural Radioactivity According to Chemical Composition in the Precambrian and Mesozoic Granitic Rocks in the Jangsu-gun Area, Central Southwestern South Korea

The Jangsu-gun area in the central Southwestern South Korea consists of a well-preserved Middle Paleoproterozoic gneissic basement, as well as the Late Triassic and Early Jurassic granitic rocks. Here, we present the detailed zircon U-Pb age data and whole-rock chemical compositions, including radioactive elements (e.g., U and Th) and activity concentrations of 226Ra, 232Th and 40K for the Middle Paleoproterozoic gneisses, and Late Triassic and Early Jurassic granitic rocks of the Jangsu-gun area. The Middle Paleoproterozoic gneissic basement, and the Late Triassic and Early Jurassic granitic rocks have ages of ca. 1988 Ma and 1824 Ma, 230 Ma and 187–189 Ma, respectively. Geochemically, the Middle Paleoproterozoic orthogneiss, Late Triassic granites and Early Jurassic granitic rocks show typical arc-related metaluminous to weakly peraluminous fractionated granite features with ASI (aluminum saturation index) values of 0.92 to 1.40. The mean values of U (ppm) and Th (ppm) of the Middle Paleoproterozoic orthogneisses (6.4 and 20.5, respectively), Late Triassic granites (1.5 and 10.9), and Early Jurassic granites (3.5 and 16.5) were similar to those (5 and 15) of the granitic rocks in the Earth’s crust. The mean 226Ra (Bq/kg), 232Th (Bq/kg), and 40K (Bq/kg) activity concentrations and radioactivity concentration index (RCI) are 62, 71, 1,214 and 0.96 for the Middle Paleoproterozoic orthogneisses; 16, 39, 1,614 and 0.78 for the Late Triassic granites; and 56, 70, 1031 and 0.88 for the Early Jurassic granitic rocks, respectively. The U, Th, 226Ra, 232Th, 40K, and RCI of the Middle Paleoproterozoic biotite paragneisses are similar to those of the Middle Paleoproterozoic orthogneisses. The trend of 226Ra, 232Th, and 40K activity concentrations, and the composition of U and Th from the Precambrian and Mesozoic rocks in the Jangsu-gun area indicates that monazite is the main accessory mineral controlling the concentration of natural radioactivity. Based on a detailed examination of the natural radioactivity in the rocks of the Jangsu-gun area, the Middle Paleoproterozoic orthogneisses and paragneisses, and Late Triassic and Early Jurassic granitic rocks show average high mean RCI values of 0.88−0.96, such that 32% of the rocks exceeded the recommended value of one in the guidelines for the RCI in South Korea. Especially, the RCI is closely related to the radon levels in the rocks. As a result, the Jangsu-gun area in South Korea is a relatively high radiological risk area, which exhibits higher indoor radon levels in the residences, compared with residences in the other areas in South Korea.

Geology of the Macao Special Administrative Region (China)

ABSTRACT A 1:12,000 geological map of the Macao Special Administrative Region has been produced through detailed field work supported by petrographic, mineralogical, geochronological and geochemical data obtained in previous studies. This map aims to represent a reliable tool to understand the geological evolution of the region and for management of the territory. The geology of Macao is dominated by two groups of Jurassic granitic rocks belonging to an intrusive suite located along the coast of Southeast China: Macao Group I (MGI: 164.5 ± 0.6 to 162.9 ± 0.7 Ma) and Macao Group II (MGII: 156.6 ± 0.2 to 155.5 ± 0.8 Ma), including the associated microgranite, aplite and pegmatite dikes and quartz veins. Remnants of the metasedimentary wall-rock are present as Devonian xenoliths enclosed within the granites. Younger Jurassic to Cretaceous andesite to dacite dikes (150.6 ± 0.6 to <120 Ma) intrude the granitic rocks. Additionally, Quaternary sedimentary deposits cover the older lithologies.

Integrated assessment of major element geochemistry and geological setting of traditional natural mineral water sources in South Korea at the national scale

Springs and hot spas have been used worldwide since ancient times as sources of potable, medicinal and bathing water, as well as for industrial purposes. They also sustain spring-dependent ecosystems and provide water flow to downstream ecosystems. We investigated the major element hydrogeochemistry of traditional springs, CO2-rich waters, and hot spas in South Korea and their relationships with features of bedrock geology and structural settings at the national scale to classify water types and identify the major factors controlling the occurrence of each water type. Three criteria—mineral content, CO2(aq), and water temperature—defined six major water types. Alkaline mineral water in carbonate rocks and sedimentary rocks result from readily weatherable lithology. The majority of mineralized waters are CO2-rich or thermal waters with a wide range of mineral contents. Their occurrence was found to be mainly concentrated near faults and boundaries between Early to Middle Jurassic granitic rocks and other rocks of much older ages, and this pattern was quantitatively identified through proximity analysis. This finding indicates that younger granitic rocks provide sources of heat and deep-seated CO2, and faults link these deep sources to groundwater flow. Hydrolysis of primary silicate minerals, including feldspar and biotite, is likely to supply major and minor solutes including Mg2+, F-, and Fe to CO2-rich and thermal waters. F- concentration appears to be regulated by the saturation states of calcite and fluorite, and Mg2+ concentration in alkaline thermal water is driven by the formation of Mg-bearing secondary silicates. We demonstrated that natural mineral water in tectonically stable regions has diverse thermal and hydrogeochemical properties that are mainly controlled by their lithological and structural settings at the regional scale. These results may help elucidate the generation processes of natural mineral waters with high cultural and ecological values, leading to their sustainable use and protection.

Co-development of Jurassic I-type and A-type granites in southern Hunan, South China: Dual control by plate subduction and intraplate mantle upwelling

Two types of spatially and temporally associated Jurassic granitic rocks, I-type and A-type, occur as pluton pairs in several locations in southern Hunan Province, South China. This paper aims to investigate the genetic relationships and tectonic mechanisms of the co-development of distinct granitic rocks through petrological, geochemical and geochronological studies. Zircon LA-ICPMS dating results yielded concordant U–Pb ages ranging from 180 to 148 Ma for the Baoshan and Tongshanling I-type granodiorites, and from 180 to 158 Ma for the counterpart Huangshaping and Tuling A-type granites. Petrologically, the I-type granodiorites consist of mafic minerals such as hornblende whereas the A-type granites are dominated by felsic minerals (e.g., quartz, K-feldspar and plagioclase). Major and trace element analyses indicate that the I-type granodiorites have relatively low SiO2 (64.5–71.0%) and relatively high TiO2 (0.28–0.51%), Al2O3 (13.8–15.5%), total FeO (2.3–4.7%), MgO (1.3–2.6%) and P2O5 (0.10–0.23%) contents, and the A-type granites are characterized by high concentrations of Rb (212–1499 ppm), Th (18.3–52.6 ppm), U (11.8–33.6 ppm), Ga (20.0–36.6 ppm), Y (27.1–134.0 ppm) and HREE (20.3–70.0 ppm), with pronounced negative Eu anomalies (Eu/Eu* = 0.01–0.15). Moreover, the I-type granodiorites are classified as collision-related granites emplaced under a compressional environment, whereas the A-type granites are within-plate granites generated in an extensional setting. Zircon Hf isotopic compositions vary substantially for these granitic rocks. The I-type granodiorites are characterized by relatively young Hf model ages (TDM1 = 1065–1302 Ma, TDMC =1589–2061 Ma) and moderately negative εHf(t) values (–5.9 to –11.5), whereas the A-type granites have very old model ages (TDM1 = 1454–2215 Ma, TDMC = 2211–2974 Ma) and pronounced negative εHf(t) values (–15.8 to –28.3). These petrochemical and isotopic characteristics indicate that the I-type granodiorites may have been derived from a deep source involving mantle-derived juvenile (basaltic) and crustal (pelitic) components, whereas the A-type granites may have been sourced from melting of meta-greywacke in the crust. This study proposes that the pressure and temperature differences in the source regions caused by combined effects of intra-plate mantle upwelling and plate subduction are the major controlling factors of the co-development of the two different types of magmas. Crustal anatexis related to lithospheric delamination and upwelling of hot asthenosphere under a high pressure and temperature environment led to the formation of the I-type magmas. On the other hand, the A-type magmas were formed from melting of the shallower part of the crust, where extensional stress was dominant and mantle-crust interaction was relatively weak. Rifts and faults caused by mantle upwelling developed from surface to depth and successively became channels for the ascending I- and A-type magmas, resulting in the emplacement of magmas in adjacent areas from sources at different depths.

Detrital zircon multi‐chronology, provenance, and low‐grade metamorphism of the Cretaceous Shimanto accretionary complex, eastern Shikoku, Southwest Japan: Tectonic evolution in response to igneous activity within a subduction zone

AbstractDetrital zircon multi‐chronology combined with provenance and low‐grade metamorphism analyses enables the reinterpretation of the tectonic evolution of the Cretaceous Shimanto accretionary complex in Southwest Japan. Detrital zircon U–Pb ages and provenance analysis defines the depositional age of trench‐fill turbidites associated with igneous activity in provenance. Periods of low igneous activity are recorded by youngest single grain zircon U–Pb ages (YSG) that approximate or are older than the depositional ages obtained from radiolarian fossil‐bearing mudstone. Periods of intensive igneous activity recorded by youngest cluster U–Pb ages (YC1σ) that correspond to the younger limits of radiolarian ages. The YC1σ U–Pb ages obtained from sandstones within mélange units provide more accurate younger depositional ages than radiolarian ages derived from mudstone. Determining true depositional ages requires a combination of fossil data, detrital zircon ages, and provenance information. Fission‐track ages using zircons estimated YC1σ U–Pb ages are useful for assessing depositional and annealing ages for the low‐grade metamorphosed accretionary complex. These new dating presented here indicates the following tectonic history of the accretionary wedge. Evolution of the Shimanto accretionary complex from the Albian to the Turonian was caused by the subduction of the Izanagi plate, a process that supplied sediments via the erosion of Permian and Triassic to Early Jurassic granitic rocks and the eruption of minor amounts of Early Cretaceous intermediate volcanic rocks. The complex subsequently underwent intensive igneous activity from the Coniacian to the early Paleocene as a result of the subduction of a hot and young oceanic slab, such as the Kula–Pacific plate. Finally, the major out‐of‐sequence thrusts of the Fukase Fault and the Aki Tectonic Line formed after the middle Eocene, and this reactivation of the Shimanto accretionary complex as a result of the subduction of the Pacific plate.

Geochronology and Petrogenesis of Mesozoic Granitoids in the Geological Corridor of Western Liaoning Province, China

Geochronology and Petrogenesis of Mesozoic Granitoids in the Geological Corridor of Western Liaoning Province, China

Geochronology and geochemistry of the major host rock of the Dong’an gold deposit, Lesser Khingan Range: Implications for petrogenesis and metallogenic setting during the Early–Middle Jurassic in northeast China

The Dong’an gold deposit is a large-sized epithermal gold deposit recently discovered in the Lesser Khingan Range, NE China. Here, we present a detailed study of the petrogenesis, magma source, and tectonic setting of a medium–coarse grained alkali-feldspar granite, the major host rock of the Dong’an gold deposit. The LA–ICP–MS zircon U–Pb dating of the medium–coarse grained alkali-feldspar granite yields an early Jurassic age of 176.3±1.1Ma (MSWD=0.62). The whole-rock geochemical data indicate that the samples are felsic, ferroan, alkali-calcic and peraluminous with relatively high alkali (K2O+Na2O) content. They are enriched in LREEs and LILEs (e.g., Rb, Ba, K), but are depleted in HFSEs (e.g., Nb, Ta, P, Ti), especially in P and Ti, showing characteristics of volcanic arc magmas and similarities with the Early–Middle Jurassic granitic rocks in Xing’an Mongolian orogenic belt. Meanwhile, the negative Eu, Nb, Ta, Ti, and P anomalies are consistent with fractional crystallization of plagioclase, Ti-bearing phases (rutile, ilmenite, titanite, etc.) and apatite during magma evolution. The samples have low Nb/Ta ratios (8.65–14.91) and low Mg# values (18–36), which are indicative of crustal derived magmas and no interaction between source magmas and the mantle. In-situ Hf isotopic analyses of the zircons from the medium–coarse grained alkali-feldspar granite yield εHf(t) values of +3.38–+5.68 and two-stage model ages (TDM2) of 772–900 Ma, indicating the magmas formed this intrusion were generated by partial melting of Neoproterozoic basaltic materials in the young lower crust, and the magma source could be derived from a depleted mantle. The medium–coarse grained alkali-feldspar granite most likely formed in the late stage of Toarcian subduction of the Pacific plate, which can be identified on the tectonic setting discrimination diagrams, and the formation of this intrusion was associated with underplating of mantle-derived magmas, which provided heat for crustal partial melting. Similar to the medium–coarse grained alkali-feldspar granite, large amounts of granitic rocks and a series of nonferrous metal hydrothermal deposits (Mo, Cu, Au) formed in northeast China as results of magmatic activities triggered by subduction of the Pacific plate during the Early–Middle Jurassic.

Provenance Analysis of Lower Cretaceous Sindong Group Sandstones in the Gyeongsang Basin, Korea, Using Integrated Petrography, Quartz Sem-Cathodoluminescence, and Zircon Zr/Hf Analysis

The Sindong Group (Aptian–Albian) in southeastern Korea is a 2–3-km-thick fluvio-lacustrine sedimentary package deposited in an elongated basin (the Nakdong Trough), which was formed by extension in an active-continental-margin setting. The provenance of the Sindong Group was studied to understand spatial and temporal variation in composition in three different parts of the basin by using integrated data on petrography, quartz SEM-cathodoluminescence (CL) analysis, and zircon Zr/Hf analysis. Although Sindong Group sandstones display wide variation in the composition of framework grains, they generally have increasing amounts of feldspar and decreasing amounts of quartz up sequence. Significant amounts of volcanic rock fragments and volcanic quartz are observed in the late-stage sediments. Metamorphic quartz is predominant in all Sindong Group sandstones, indicative of the exposure of metamorphic rocks in the source terrane, mostly from Precambrian basement and Triassic granites. The occurrence of significant amounts of plutonic quartz from early-stage sediments suggests that Jurassic granites were widely exposed in the source terranes. In addition, the episodic increase in volume of plutonic quartz content in the sequence, especially in the northern part of the basin, suggests that episodic tectonic activity occurred in the catchment. The increasing feldspar content up sequence could support the occurrence of tectonic activity in the catchments. The Zr/Hf analysis of detrital zircons revealed that the majority of zircons are of continental-crust origin formed in orogenic settings, but zircons in the lowest strata in the northern part of the basin were derived largely from anorogenic magmatic rocks of mantle origin. Our results demonstrate that source terranes for the Sindong Group have heterogeneous spatial and temporal distribution and were composed mainly of Precambrian basement and Triassic to Jurassic granitic rocks, with minor (meta)sedimentary rocks and syndepositional volcanic rocks. The detritus derived from syndepositional volcanic rocks became significant in the late stage of basin filling, representing the transition from extensional tectonism to continental-arc magmatism. The differences in characteristics of quartz SEM-CL and zircon Zr/Hf ratios in different parts of the basin are best explained by deposition on different alluvial fans and river systems.

Petrochemistry of igneous rocks of the California-Vetas mining district, Santander, Colombia: Implications for northern Andean tectonics and porphyry Cu (–Mo, Au) metallogeny

Porphyry Mo and Cu mineralization in the California-Vetas mining district is contemporaneous with 10.9 to 8.4 Ma granodiorite porphyry stocks and overprinted by Au–Ag mineralization of epithermal affinity. Mineralization is hosted by Grenvillian aged paragneisses (Bucaramanga Gneiss of the Santander Massif) and late Triassic to early Jurassic granitic rocks. All intrusive rocks are high-K calc-alkaline. Late Triassic to early Jurassic rocks include peraluminous granites with more than 70 wt.% SiO2 as well as metaluminous diorites, tonalites and granodiorites with SiO2 between 54.9 and 60.4 wt.%. Late Miocene rocks are weakly peraluminous granodiorite porphyries with SiO2 between 61 and 67 wt.% SiO2. Late Miocene rocks share some characteristics with adakite-like rocks which are widely associated with porphyry and epithermal style mineralization elsewhere in the Andes. They have high Ba (930 to 1500 ppm) and high Ba/La (28 to 50), high Sr (850 to 1100 ppm) and Sr/Y (48–78) and depleted middle rare earth elements (MREE) compared to the Mesozoic granites, which have 400 to 700 ppm Ba (Ba/La 14 to 25) and 80 to 150 ppm Sr (Sr/Y 2.5 to 14), and Mesozoic diorites and tonalites, which have ~ 900 to 1200 ppm Ba (Ba/La 20 to 32) and ~ 610 to 750 ppm Sr (Sr/Y 22 to 25). Miocene granodiorite porphyries, in contrast to Mesozoic intrusive rocks have only weak negative Eu anomalies. The Miocene rocks have 87Sr/86Sr ratios of 0.7052 to 0.7067 and εNd of − 1.9 to − 5.4 and are significantly more isotopically primitive than all other rocks in the study area including the Mesozoic diorites to tonalites (87Sr/86Sr = 0.7082 and 0.7092; εNd = − 6.7 and − 7.2), granites (87Sr/86Sr = 0.730 (n = 2); εNd = − 8.2 and − 8.3) and Bucaramanga Gneiss (0.718 to 0.743; εNd = − 10.8 to − 14.1). Lead isotope data are broadly consistent with the Sr and Nd isotope data and the Miocene porphyries have the lowest 207Pb/204Pb ratios but overlap with the Mesozoic diorites to tonalites in their 206Pb/204Pb and 208Pb/204Pb ratios. The geochemical signature suggests that late Miocene granodiorite porphyries represent oxidized and water-rich melts. However, in contrast to many other porphyry districts in the Andes, they do not have low Y (< 15 ppm) or depleted HREE, which together with the high Sr, Al and Na content indicates that neither plagioclase nor garnet were important residual phases in the MASH zone at the base of the crust. The trough shaped MREE suggest that amphibole was present in the source. The 87Sr/86Sr values of the Miocene rocks are consistent with a mantle derived melt which interacted with the radiogenic continental crust. The magmatism occurred above the shallow subducting Caribbean slab attributed to subduction of part of the Caribbean plateau (a.k.a. proto-Caribbean oceanic complex). It may be related to slab break-off and incursion of hot asthenospheric mantle, but dehydration of the Caribbean slab probably provided additional volatiles which made amphibole a major residual phase and provided water for the hydrous porphyry related magmas.

On the petrolochemical characteristics of Jurassic granitic rocks intruded in the Suan copper mining district, Korean Peninsula

On the petrolochemical characteristics of Jurassic granitic rocks intruded in the Suan copper mining district, Korean Peninsula

Characteristic Features of Tin–iron–copper Mineralization in the Anle‐Huanggangliang Mining Area, Inner Mongolia, China

Abstract: The Anle Sn‐Cu and Huanggangliang Fe‐Sn deposits have been exploited in the Linxi district, which is located 165 km northwest of Chifeng City in northern China. In this study the formation mechanisms of the tin deposits in the Anle and Huanggangliang mining area were investigated to understand the mechanisms of tin mineralization in northern China.The veins of the Anle deposit are divided into cassiterite–quartz–chlorite veins, chalcopyrite‐bearing quartz veins, cassi–terite–chalcopyrite–bearing quartz veins and sphalerite‐quartz veins. The sequence of mineralization is tin mineralization (stage I), copper mineralization (stage II), and lead‐zinc mineralization (stage III). The Huanggangliang tin deposit consists of magnetite skarn orebodies and many cassiterite‐bearing feldspar–fluorite veins and veinlets cutting the magnetite orebodies.The fluid inclusions in quartz and fluorite in ores from the Anle and Huanggangliang tin deposits are divided into two‐phase fluid inclusions, vapor‐rich fluid inclusions and poly‐phase fluid inclusions. The final homogenization temperatures of fluid inclusions of quartz in the ores of the Anle deposit and fluorite of tin‐bearing feldspar veins in the Huanggangliang tin deposit range from 195 to 425C and from 215 to 450C, respectively. The fluids responsible for the Anle and Huanggangliang tin deposits were of very high temperature and NaCl‐rich ones containing K, Ca, Al, Si, Ti, Fe and Cl in addition to ore metals such as Sn and Cu. The temperature and chemical composition of fluid in fluid inclusions of igneous rocks in the mining area are very similar to those of fluid in fluid inclusions in the ores of these deposits. The fluid for these ore deposits had a close relation with the fluid coexisting with melt of Late Jurassic granitic rocks in this mining area. Salinities of fluid inclusions from these ore deposits and granitic rocks in the mining area were estimated to range from 35 to 50 wt % NaCl equivalent. Based on arsenopy‐rite geothermometry and fluid inclusion studies, a fluid containing 40 wt% NaCl (eq.) could be formed by phase separation of fluid having 6 wt% NaCl (eq.) at a temperature of 420 to 500C and a pressure of 0.3 to 0.4 kb. The temperatures and pressures presented above indicate an NaCl‐rich magmatic fluid derived from granitic melt that had intruded into a shallow level of crust caused the Sn–Fe–Cu mineralization of the mining area. The geological relationship between these ore deposits and granitic bodies around the ore deposits, and the similarity of fluids forming these ore deposits and coexisting with granitic melt, suggest that these ore deposits were formed by the activity of fluid derived from granitic melt in Late Jurassic age.

Two types of granitic intrusions in the Hida belt, Japan: Sr isotopic and chemical characteristics of the Mesozoic Funatsu granitic rocks

Initial 87 Sr 86 Sr ratios, major- and trace-element compositions have been determined for the Late Triassic to Early Jurassic granitic rocks (Funatsu granitic rocks) in the Hida belt, Japan. Six intrusions, which are exposed in a transverse section of the belt, were selected for analyses. Based on the characteristics of initial 87 Sr 86 Sr ratios (Sr I-values) and element concentrations, the above intrusions are classified into two types. One is the intrusion with nearly constant Sr I-values over the variation of rock compositions. The Utsubo (Sr I=0.7053) and Hirose (0.7044) intrusions in this study, and the already reported Funatsu and Shimonomoto intrusions (both 0.7048) belong to this group, and rocks in this group are interpreted to have been formed mainly by fractional crystallization from mafic magma that originated in the lower crust or upper mantle. In contrast, the rocks in the other four intrusions (Hodatsusan, Shokawa, Yatsuo and Nagarehayama) show relatively higher and variable Sr I-values (0.7047–0.7105). Two-stage models, crustal mixing by the parental mafic magma at mid-crustal levels and subsequent differentiation by fractional crystallization, are interpreted to have been the main mechanisms for producing granitic rocks in these intrusions. Two contrasting types of Sr isotopic and chemical compositions recognized in the intrusions of the Funatsu granitic rocks, which are attributed to a difference of the predominant mechanism (crustal mixing or differentiation), correlate well with two geological features observed in the Hida belt. One is the existence or absence of shear zones which contemporaneously formed with granitic emplacement, and the other is the pattern of chemical zonation of the intrusion. Intrusions with a wide range of high Sr I-values are accompanied by a dextral shear zone in one margin and show a reverse- (less normal-) type chemical zonation. On the other hand, intrusions with constant Sr I-values do not relate with shear deformation and have normal chemical zonation. Considering these correlations, it is concluded that the stress state in middle-to upper-crustal levels is one of the important factors controlling the mechanisms of magma formation and way of emplacement.

Igneous history of the Koyukuk Terrane, western Alaska: Constraints on the origin, evolution, and ultimate collision of an accreted island arc terrane

The Koyukuk terrane of western Alaska consists of volcanic, volcaniclastic, and plutonic rocks which range from Late Paleozoic to Early Cretaceous in age. The terrane crops out in a U‐shaped belt which is roughly paralleled by outer belts of ultramafic rocks, oceanic plate basalts and cherts, and retrograded blueschist facies rocks of continental protolith. These rocks have been interpreted as components of a volcanic arc terrane that collided with the North American continental margin in Early Cretaceous time. The Koyukuk terrane consists of four time‐stratigraphic units: (1) pre‐Middle Jurassic basalts, (2) Middle and Late Jurassic granitic rocks, (3) lower Lower Cretaceous volcanic rocks, and (4) upper Lower Cretaceous volcanic rocks. Limited chemical data from the basalts of unit 1 indicate that they were erupted in a nonarc tectonic environment, possibly in an oceanic island or back arc setting. Units 2, 3, and 4 have the characteristics of subduction‐related volcanic rocks (i.e., depleted Nb and Ta and enriched alkaline elements, relative to the light rare earth elements). Unit 3 contains tholeiitic, calc‐alkaline, and alkaline rocks with chondrite‐normalized rare earth element patterns that range from flat (LaN/YbN = 1) to highly light rare earth element enriched (LaN/YbN &gt; 15). The highly alkaline or shoshonitic lavas were erupted toward the end of unit 3 time (Valanginian) during the final stages of arc‐continent collision. These alkaline lavas could have been derived by very small degrees of partial melting of a similar source to that of the earlier arc lavas. Unit 4 lavas are also alkaline or shoshonitic, but their incompatible element composition indicates that they were derived from a different source than that of the earlier arc lavas. These late alkaline lavas are chemically similar to crosscutting mid‐Cretaceous plutons whose isotopic compositions (Arth et al., this issue (a)) suggest derivation by partial melting of distinctly older subcontinental lithosphere. We speculate that the parental magmas of unit 4 lavas may also have been derived by partial melting of this subcontinental mantle which was underthrust beneath the Koyukuk arc terrane during the final stage of arc‐continent collision.

Age and Rate of Movement of the Northern Part of the Death Valley-Furnace Creek Fault Zone, California

Large right-lateral displacements along the Death Valley-Furnace Creek fault zone have probably taken place since Middle Jurassic time. Matching of Middle Jurassic granitic rocks (160 m.y. old by K-Ar dating) on either side of the fault zone suggests displacement on the order of 30 miles. The distribution of Pliocene sedimentary rocks (3.5 m.y. old by K-Ar dating) in the same region suggests the possibility of approximately 3000 feet of right-lateral movement since Pliocene time. The calculated rate of displacement since the Middle Jurassic is similar to the tentative rate since the late Pliocene: