Luzong Basin Research Articles

APPLICATION OF AN ENHANCED THETA‐BASED FILTER FOR POTENTIAL FIELD EDGE DETECTION: A CASE STUDY OF THE LUZONG ORE DISTRICT

AbstractEdge detection plays an important role in interpreting potential field data, and is widely used to delineate geologic boundaries and structures. Geologic boundaries can be determined by tracing the enhanced analytical signal, and many filters are developed to detect and enhance the edges. Horizontal and vertical derivatives are commonly used to enhance edge features, but they can only outline the edges of large‐amplitude anomalies. In order to display large and small amplitude anomalies simultaneously, some balanced filters have been proposed. We define new filters based on the Theta map method, using 2nd order horizontal and vertical directional derivatives, and display large and small amplitude edges simultaneously. These methods were tested on synthetic gravity data, and compared with other traditional filters; the results show that the new filters can achieve better results and reveal more details. The method has been applied to gravity‐magnetic data acquired in the Luzong ore district in the Middle‐Lower Yangtze River Valley Metallogenic Belt (MLYB, Eastern China). Based on the relations between lithology, density and magnetic susceptibility, the calculated results were analyzed. The edge detection results accurately depict the location of the Tanlu fault zone. A gravity boundary north of the Yangtze River is interpreted to be part of the Yangtze River fault. The identified boundaries from the magnetic data are consistent with the margins of the Luzong basin, and indicate that the bounding faults dip towards the basin. Some ring‐like closed boundaries occur in the periphery of the Luzong basin. Recent exploration results confirm that there are buried intrusions beneath the gravity‐magnetic anomaly bodies, where Fe‐Cu skarn mineralization is present. The results of this study provide significant insights for regional deep‐level Fe‐Cu exploration.

Geochronology and petrogenesis of shoshonitic igneous rocks, Luzong volcanic basin, middle and lower Yangtze River reaches, China

The Luzong volcanic basin has a unique tectonic background, being located at the northeastern margin of the Yangtze craton and the foreland of the Dabie–Sulu orogen, near the intersection of the renowned Tanlu and Yangtze River faults. The volcanic and subvolcanic rocks in the basin are characterized by high potassium contents and are rich in alkali elements, and belong to a typical shoshonite series. Geochemically, these rocks are depleted in high field strength elements such as Nb and Ta, and enriched in highly incompatible elements such as Rb, Th, U, K, and light rare earth elements. Nd and Sr isotope compositions, combined with elemental geochemical characteristics, indicate that the parental magmas were formed mainly by partial melting of enriched subcontinental lithospheric mantle. The magmas underwent minor or negligible contamination by upper crustal rocks or fractional crystallization at low pressures (i.e., <1.5GPa). Nevertheless, variation trends for major and trace elements indicate that these shoshonitic mantle-derived magmas experienced fractional crystallization at high pressures (i.e., above the stability field of plagioclase, >1.5GPa), dominated by the removal of clinopyroxene and Fe–Ti oxides. The volcanic rocks of the Luzong Basin can be divided into four formations, which, from early to late, are the Longmenyuan, Zhuanqiao, Shuangmiao, and Fushan Formations, each separated by clear gaps in eruptive activity. In this study, we obtained zircon U–Pb SHRIMP ages from two subvolcanic samples from the Longmenyuan and Zhuanqiao Formations, which yield emplacement ages of 132±1Ma and 131±1Ma. These data, combined with ages reported by other workers, show that the duration of volcanism in the Luzong Basin (and the whole middle and lower Yangtze River reaches) was very short (⩽8Ma). We consider that in the Early Cretaceous the foreland area of the Dabie orogenic belt was under extension, influenced by the relative motion of the Pacific and Eurasian plates, and by sinistral displacement along the Tanlu Fault. At this time, rapid mechanical delamination of thickened lithosphere caused lithospheric thinning and resultant asthenospheric upwelling in the middle and lower Yangtze River region and the Dabie orogenic belt. These deep geological and tectonic events in turn produced the shoshonite volcanic rocks in the Luzong Basin.

Basalt–water interactions at high temperatures: 1. Dissolution kinetic experiments of basalt in water and NaCl-H2O at temperatures up to 400 °C, 23 MPa and implications

Kinetic experiments of basaltic rock–water (and NaCl-H2O) interactions at temperatures from 20 to 400°C were carried out using flow through a backed bed reactor. Experimental results indicate that all of the metal release rates vary with temperature and that the relative release rates for the various metals in the basaltic rock vary significantly at constant temperature. The rock dissolution rate, i.e., the release rate of Si, increased as the temperature increased from 20°C to 300°C, and then decreased as the temperature continued to increase from 300°C to 400°C. The release rate of Si reached a maximum at 300°C (at 350°C in NaCl-H2O). The maximum release rate of Al was reached at 350°C. Release rates of Ca, Mg and Fe decreased with increasing temperature from 20 to 300°C (or 350°C), and generally, Ca, Mg, Fe, and Al were dissolved more quickly than Si at temperatures in the range from 25 to 300°C, but more slowly in the range from 300 to 400°C. Our experiments demonstrated that Si will be fixed on the rock surface at temperatures from 100 to 300°C, Al will be fixed at 200–300°C, or >350°C; but Mg, Ca and Fe will be fixed on the surface at temperatures from 300 to 400°C. The experimental results can be used to illustrate the factors controlling alteration zoning in sub-sea floor rocks at Mid-Ocean Ridges (MOR) and in continental volcanic rocks in the Luzong basin, Yangtze river valley, China. The results also demonstrate that alteration zoning in basaltic rocks in a geological profile with a temperature gradient is caused by the variable release rates for different metals with temperature. Strong variations in those release rates always occurred at 300–400°C, which reflect changes in the properties of water as it passes from a sub-critical to a supercritical state.

The role of evaporites in the formation of magnetite–apatite deposits along the Middle and Lower Yangtze River, China: Evidence from LA-ICP-MS analysis of fluid inclusions

Numerous magnetite–apatite deposits occur in the Ningwu and Luzong sedimentary basins along the Middle and Lower Yangtze River, China. These deposits are located in the contact zone of (gabbro)-dioritic porphyries with surrounding volcanic or sedimentary rocks and are characterized by massive, vein and disseminated magnetite–apatite±anhydrite mineralization associated with voluminous sodic–calcic alteration. Petrologic and microthermometric studies on multiphase inclusions in pre- to syn-mineralization pyroxene and garnet from the deposits at Meishan (Ningwu basin), Luohe and Nihe (both in Luzong basin) demonstrate that they represent extremely saline brines (~90wt.% NaClequiv) that were trapped at temperatures of about 780°C. Laser ablation ICP-MS analyses and Raman spectroscopic studies on the natural fluid inclusions and synthetic fluid inclusions manufactured at similar P–T conditions reveal that the brines are composed mainly of Na (13–24wt.%), K (7–11wt.%), Ca (~7wt.%), Fe (~2wt.%), Cl (19–47wt.%) and variable amounts of SO4 (3–39wt.%). Their Cl/Br, Na/K and Na/B ratios are markedly different from those of seawater evaporation brines and lie between those of magmatic fluids and sedimentary halite, suggesting a significant contribution from halite-bearing evaporites. High S/B and Ca/Na ratios in the fluid inclusions and heavy sulfur isotopic signatures of syn- to post-mineralization anhydrite (δ34SAnh=+15.2 to +16.9‰) and pyrite (δ34SPy=+4.6‰ to +12.1‰) further suggest a significant contribution from sedimentary anhydrite. These interpretations are in line with the presence of evaporite sequences in the lower parts of the sedimentary basins.The combined evidence thus suggests that the magnetite–apatite deposits along the Middle and Lower Yangtze River formed by fluids that exsolved from magmas that assimilated substantial amounts of Triassic evaporites during their ascent. Due to their Fe-oxide dominated mineralogy, their association with large-scale sodic–calcic alteration and their spatial and temporal associations with subvolcanic intrusions we interpret them as a special type of IOCG deposits that is characterized by unusually high contents of Na, Ca, Cl and SO4 in the ore-forming fluids. Evaporite assimilation apparently led to the production of large amounts of high-salinity brine and thus to an enhanced capacity to extract iron from the (gabbro)-dioritic intrusions and to concentrate it in the form of ore bodies. Hence, we believe that evaporite-bearing sedimentary basins are more prospective for magnetite–apatite deposits than evaporite-free basins.

Genesis and Prospecting of the Uranium and Thorium Mineralization Indicated from Deep Drilling ZK01, Luzong Basin

Acta Geologica Sinica - English EditionVolume 88, Issue s2 p. 1402-1403 Meeting Abstracts Genesis and Prospecting of the Uranium and Thorium Mineralization Indicated from Deep Drilling ZK01, Luzong Basin Xin XIONG, Corresponding Author Xin XIONG MLR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 100037 ChinaCorresponding author. E-mail: XiongXin_1989@163.comSearch for more papers by this authorZhusen YANG, Zhusen YANG MLR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 100037 ChinaSearch for more papers by this authorWenyi XU, Wenyi XU MLR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 100037 ChinaSearch for more papers by this authorLiqiong JIA, Liqiong JIA MLR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 100037 China School of Earth Science and Mineral Resources, China University of Geosciences, Beijing 100083 ChinaSearch for more papers by this authorJun LI, Jun LI MLR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 100037 China School of Earth Science and Mineral Resources, China University of Geosciences, Beijing 100083 ChinaSearch for more papers by this author Xin XIONG, Corresponding Author Xin XIONG MLR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 100037 ChinaCorresponding author. E-mail: XiongXin_1989@163.comSearch for more papers by this authorZhusen YANG, Zhusen YANG MLR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 100037 ChinaSearch for more papers by this authorWenyi XU, Wenyi XU MLR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 100037 ChinaSearch for more papers by this authorLiqiong JIA, Liqiong JIA MLR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 100037 China School of Earth Science and Mineral Resources, China University of Geosciences, Beijing 100083 ChinaSearch for more papers by this authorJun LI, Jun LI MLR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 100037 China School of Earth Science and Mineral Resources, China University of Geosciences, Beijing 100083 ChinaSearch for more papers by this author First published: 29 December 2014 https://doi.org/10.1111/1755-6724.12381_31Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat No abstract is available for this article. Volume88, Issues2Special Issue: Meeting Abstracts: The 14th Quadrennial International Association on the Genesis of Ore Deposits Symposium. August 19–22, 2014, Kunming, ChinaDecember 2014Pages 1402-1403 RelatedInformation

Application of apatite U–Pb and fission-track double dating to determine the preservation potential of magnetite–apatite deposits in the Luzong and Ningwu volcanic basins, eastern China

The Cretaceous Luzong and Ningwu volcanic basins in eastern China contain numerous magnetite–apatite deposits with similar geological characteristics and mineralization ages (~130Ma). These deposits, however, occur at distinctly different burial depths. To explain this difference, LA-ICP-MS apatite U–Pb and fission track double dating of five samples were carried out to establish the thermal histories from crystallization to the exhumation of four representative deposits: the deeply buried Nihe (665–1065m underground) and Luohe deposits (425–856m underground) from the Luzong basin; the shallow Meishan deposit (40–530m underground) and the exposed Dongshan deposit from the Ningwu basin. The cooling histories of the four deposits could be divided into rapid cooling for the Dongshan deposit and slow continuous cooling for the Nihe, Luohe, and Meishan deposits. Combined with geological evidence, it can be determined that the Dongshan and Meishan deposits formed at shallow depths with the Dongshan deposit emplaced at a very high level while the Meishan deposit was emplaced relatively deeper. The Nihe and Luohe deposits were emplaced deeper than the Dongshan and Meishan deposits and the burying of the Shuangmiao volcanic cycle after mineralization increased the burial depth. However, this burial event did not occur in the Ningwu basin. All of these resulted in the different burial of the magnetite–apatite deposits between the two basins prior to 120Ma while the exhumation after 120Ma among these deposits was similarly slow and continuous. The metallogenic conditions of both basins were similar but the smaller number and gross reserve of magnetite–apatite deposits in the Luzong basin, we believe that the Luzong basin has better preservation potential for the magnetite–apatite deposits.

Origin of andesitic rocks: Geochemical constraints from Mesozoic volcanics in the Luzong basin, South China

A combined study of whole-rock major-trace elements and Sr–Nd–Pb–Hf isotopes as well as zircon U–Pb ages and Hf–O isotopes was carried out for Mesozoic andesitic–basaltic volcanics from the Luzong basin in the Middle–Lower Yangtze River Belt, South China. The results provide insights into the origin of mantle sources above fossil Andes-type oceanic subduction zone and thus into the petrogenesis of andesitic rocks on fossil and modern continental margins. These volcanics are primarily composed of basaltic trachyandesite and trachyandesite, with small amounts of trachybasalt and trachyte. They exhibit variable contents of SiO2 (48.66–63.43wt.%), MgO (0.39–4.85wt.%), Na2O (1.22–6.07wt.%) and K2O (2.53–10.10wt.%), with highly variable K2O/Na2O ratios from 0.45 to 7.39. They are characterized by arc-like trace element distribution patterns, with significant enrichment of LILE, Pb and LREE but depletion of HFSE. They exhibit relatively enriched Sr–Nd–Pb–Hf isotope compositions, with initial 87Sr/86Sr ratios of 0.7050 to 0.7066, negative εNd(t) values of −8.0 to −3.1, negative εHf(t) values of −11.1 to −1.1, and elevated 207Pb/204Pb and 208Pb/204Pb ratios at given 206Pb/204Pb ratios. Zircon U–Pb dating yields consistent ages of 127±2 to 137±1Ma for magma emplacement through volcanic eruption. The zircon exhibits slightly high δ18O values of 5.3 to 7.6‰ and variable εHf(t) values of −13.1 to 2.6. An integrated interpretation of all these geochemical data leads to the conclusion that the Luzong andesitic–basaltic volcanics were primarily derived from partial melting of fertile and enriched, mafic–ultramafic mantle sources that are similar to those of continental arc andesites. Such mantle sources are hypothesized to form by reaction of the mantle wedge peridotite not only with hydrous felsic melts derived from partial melting of seafloor sediment but also with aqueous fluid derived from metamorphic dehydration of altered oceanic basalt during subduction of the fossil Andes-type oceanic crust beneath continental margin. As a consequence, the mantle wedge would be metasomatized by larger amounts of the felsic melts than the case for oceanic arc basalts, yielding less ultramafic lithologies for the andesitic to basaltic magmatism. Therefore, the significant incorporation of sediment-derived felsic melts into the mantle wedge is likely a key premise to the origin of andesitic rocks in fossil and modern continental margins.

Petrology, geochemistry, and tectonic significance of Mesozoic shoshonitic volcanic rocks, Luzong volcanic basin, eastern China

We studied the geochemical characteristics of three types of Mesozoic igneous rocks from the Luzong volcanic basin: basaltic trachyandesite at Shuangmiao, pyroxene monzonite at Bajiatan, and quartz-syenite (A-type granite) at Huangmeijian. Based on analyses of whole-rock major elements, all investigated rocks are enriched in K, Na, Ti, Al, but depleted in Ca, representing a shoshonitic series. Trace element analyses show that these rocks are characterized by enrichments of large-ion lithophile elements and high field strength elements. Positive Nb and Ta anomalies in the chondrite-normalized spider diagram indicate that the shoshonitic volcanic rocks share similar features with Nb-enriched basalts, which are different from normal island-arc volcanical rocks (they are typically strongly depleted in Nb and Ta). Bulk-rock chemical compositions and Sr–Nd isotopes indicate that the three types of igneous rocks are geochemically comagmatic, suggesting that the melts were derived from an enriched mantle reservoir. We postulate an extensional tectonic setting for the formation of Luzong volcanic basin, possibly related to subduction of a palaeo-Pacific plate beneath the east Chinese continent during the Yanshanian period (Cretaceous). Therefore, the petrogenetic features of those volcanic rocks as well as A-type granites in the Luzong basin indicate that the regional large-scale Fe–Cu–Au mineralization was associated with oceanic slab melting, but not delamination or recycling of the ancient lower continental crust, as previously proposed.

Neoproterozoic subducted materials in the generation of Mesozoic Luzong volcanic rocks: Evidence from apatite geochemistry and Hf–Nd isotopic decoupling

A combined study including apatite geochemistry, zircon U–Pb, Lu–Hf isotopes and whole-rock geochemistry including Nd isotopes was carried out for the late Mesozoic volcanic rocks from the Luzong Basin, in the lower Yangtze River region, South China. Whole-rock geochemistry indicates the enrichments of large ion lithophile elements (LILE) and light rare earth elements (LREE) as well as depletions of Nb, Ta and Ti. The extremely low Cl contents in apatites strongly contrast with the rather high-K contents in whole rocks. Potential loss of Cl during syn- and post-magmatic processes having been ruled out, Cl–K decoupling is attributed to be a feature inherited from the primary magma, which indicates the involvement of highly dehydrated sediments and altered oceanic crust in the mantle source. A calculation based on apatite and whole-rock geochemistry further illustrates that the source was composed of four end-members in the perspective of Cl/K, Cl/Nb and F/K ratios. The Hf–Nd isotopes are decoupled for the basaltic trachytes from the lower volcanic sequences in the Luzong Basin, with rather low ε Hf (t) values (mean = − 10.3) and inconsistent Hf–Nd model ages (Hf ~ 1.8 Ga, Nd ~ 1.3 Ga), which indicate the “zircon effect” that in turn requires the incorporation of continental detritus in the source via subduction. However, Hf and Nd isotopes are nearly coupled for the rocks from the upper volcanic sequences in the Luzong Basin. Late-Mesoproterozoic two-stage Hf and Nd model ages (ca. 1.2 Ga) of rocks from the upper volcanic sequences in the Luzong Basin are similar to those of the Neoproterozoic igneous rocks from the Jiangnan orogen, suggesting their relationship with the same subduction event. Based on the combined apatite geochemistry and Hf–Nd isotopes, this work suggests that the source of Luzong volcanic rocks might incorporate Neoproterozoic subducted slab fragments and detrital sediments that had been blocked in the deep lithospheric mantle below the Luzong area since the Neoproterozoic assembly between the Yangtze and Cathaysia blocks. The partial melting may be triggered by the back-arc lithospheric extension related to the subduction of Paleo-Pacific plate in the late Mesozoic.

Geological, geochemical characteristics and isotope systematics of the Longqiao iron deposit in the Lu-Zong volcano-sedimentary basin, Middle-Lower Yangtze (Changjiang) River Valley, Eastern China

The Middle-Lower Yangtze (Changjiang) River Valley metallogenic belt is located on the northern margin of the Yangtze Craton of eastern China. Most polymetallic deposits in the Changjiang metallogenic belt are clustered in seven districts where magmatism of Mesozoic age (Yanshanian tectono-thermal event) is particularly extensive. From west to east these districts are: E-dong, Jiu-Rui, Anqing-Guichi, Lu-Zong, Tong-Ling, Ning-Wu and Ning-Zhen. World-class iron ore deposits occur in the Lu-Zong and Ning-Wu ore clusters, which are mainly located in continental fault-bound volcanic-sedimentary basins. One of these deposits is the Longqiao iron deposit, discovered in the northern part of the Lu-Zong Basin in 1985. This deposit consists of a single stratabound and stratiform orebody, hosted in sedimentary carbonate rocks of the Triassic Dongma'anshan Formation. A syenite pluton (Longqiao intrusion) is situated below the deposit. The iron ore is massive and disseminated and the ore minerals are mainly magnetite and minor pyrite. Wall rock alteration mostly consists of skarn minerals, such as diopside, garnet, potassic feldspar, quartz, chlorite, phlogopite and anhydrite. Thin sedimentary siderite beds of Triassic age occur as relict laminated ore at the top and the margin of the magnetite orebody. These sideritic laminae are part of Triassic evaporite-bearing carbonate deposits (Dongma'anshan Formation). Sulfur isotopic compositions show that the sulfur in the deposit was derived from a mixture of magmatic hydrothermal fluids and carbonate–evaporite host rocks. Similarly, the C and O isotopic compositions of limestones from the Dongma'anshan Formation indicate that these rocks interacted with magmatic hydrothermal fluids. The O isotopic compositions of the syenitic rocks and minerals from the deposit show that the hydrothermal magnetite and skarn minerals were formed from magmatic fluids. The Pb isotopic compositions of sulfides are similar to those of the Longqiao syenite. Phlogopite coexisting with magnetite in the magnetite ores yielded a plateau age of 130.5 ± 1.1 Ma (2 σ), whereas the LA-ICP MS age of the syenite intrusion is 131.1 ± 1.5 Ma, which is slightly older than the age of phlogopite. The Longqiao syenite intrusion may have crystallized from a parental alkaline magma, generated by partial melting of lithospheric mantle, during extensional tectonics. The ore fluids were probably first derived from magma at depth, later emplaced in the sedimentary rocks of the Dongma'anshan Formation, where it interacted with siderite and evaporite-bearing carbonate strata, resulting in the formation of magnetite and skarn minerals. The Longqiao iron deposit is a skarn-type stratabound and stratiform mineral system, genetically and temporally related to the Longqiao syenite intrusion. The Longqiao syenite is part of the widespread Mesozoic intracontinental magmatism (Yanshanian event) in eastern China, which has been linked to lithospheric delamination and asthenospheric upwelling.

Petrogenesis of volcanic and intrusive rocks of the Zhuanqiao stage, Luzong Basin, Yangtze metallogenic belt, east China: implications for ore deposition

The Mesozoic Luzong volcanic basin is located in the Lower Yangtze River fault-depression, along the northern margin of the Yangtze Block. Volcanic and plutonic rocks are widely distributed in the basin and are spatially related to Cu, Fe, Au, Pb, and Zn ore deposits. The magmatic rocks are dominated by an alkali-rich shoshonitic suite of volcanics and intrusives, including both a trachybasalt-basaltic trachyandesite-trachyte series and a diorite-monzonite-syenite-alkali feldspar granite series. Volcanic sequences within the Luzong Basin are subdivided into four stages, namely the Longmenyuan, Zhuanqiao, Shuangmiao, and Fushan stages with magmatism occurring between 136 and 124 Ma. Most mineral deposits were formed during the Zhuanqiao stage from about 134 to 131 Ma, during a longer (140–130 Ma) period of transition from compression to extension within the Luzong Basin. Major element concentrations in the Zhuanqiao volcanic rocks range from 53.97 to 62.59 wt.% SiO2, 0.57–0.94 wt.% TiO2, 3.52–6.07 wt.% Na2O, and 3.60–8.66 wt.% K2O. Combined K2O + Na2O totals lie between 7.87 and 12.49 wt.%, and K2O + Na2O/Al2O3 ranges between 0.47 and 0.71. In comparison, the Zhuanqiao intrusions have SiO2 concentrations between 52.19 and 67.70 wt.%, TiO2 between 0.26 and 1.13 wt.%, Na2O between 1.33 and 6.55 wt.%, and K2O between 2.15 and 8.25 wt.%, with K2O + Na2O values ranging from 5.92 to 12.39 wt.% and K2O + Na2O/Al2O3 ratios between 0.37 and 0.75. The Zhuanqiao volcanic rocks have an average ∑REE (rare earth element) content of 223.17 ppm, whereas the average ∑REE of the Zhuanqiao intrusives is 250.06 ppm. Weak negative Eu anomalies (Eu/Eu* between 0.69 and 1.00) characterize the volcanics, whereas a larger range with more significant negative Eu anomalies (Eu/Eu* values of 0.35–0.94) is present in the plutonics. The Zhuanqiao volcanic rocks and plutons have (Nb/Th)PM ratios of 0.05–0.14 and 0.02–0.14 and (La/Sm)PM of 3.33–4.95 and 3.36–7.29, respectively. Initial 87Sr/86Sr and 143Nd/144Nd ratios and ϵSr(t) and ϵNd(t) of the Zhuanqiao volcanic rocks are 0.7059–0.7067, 0.51194–0.51218, 22.0–33.3, and −10.2 to −5.5, respectively; those of the intrusives are 0.7060–0.7082 and 0.51203–0.51214 and 23.3–54.7 and −8.6 to −6.4, respectively. The geochemistry presented here suggests that both volcanic and intrusive Zhuanqiao stage magmatic rocks were cogenetic and were probably derived from an enriched mantle (EM) source, most probably metasomatized mantle relating to the EM type-I (EM-I). The Zhuanqiao stage magmatism apparently was controlled by differentiation and assimilation of crustal material in a high-level magma chamber. Formation of the Zhuanqiao stage magmatic rocks coincided with the formation of much of the Cu and Fe mineralization within the Luzong Basin. This coeval magmatism and mineralization during the Zhuanqiao stage can be subdivided into three differing mineral deposit affinities: magmatism associated with magmato-hydrothermal vein Cu (e.g. the Jingbian, Shimenan, and Chuanshandong deposits), porphyry-hosted Fe (e.g. the Luohe and Nihe deposits), and skarn Fe (e.g. the Longqiao deposit). Combining geological characteristics with variations in SiO2, K2O + Na2O, and MgO enabled the division of the Zhuanqiao stage magmatism in the Luzong Basin into geochemically distinct units associated with these differing styles of mineralization.

Geochronology of the volcanic rocks in the Lu-Zong basin and its significance

The Lu-Zong (Lujiang-Zongyang) basin is one of the most important volcanic basins in the middle and lower reaches of the Yangtze River area, China. It comprises four shoshonitic volcanic units, which are, in an ascending order, the Longmenyuan, Zhuanqiao, Shuangmiao and Fushan Groups. The LA-ICP MS U-Pb zircon ages of the four units are: 134.8±1.8 Ma for the Longmenyuan Group, 134.1±1.6 Ma for the Zhuanqiao Group, 130.5±0.8 Ma for the Shuangmiao Group, and 127.1±1.2 Ma for the Fushan Group. The results indicate that all volcanic rocks in the Lu-Zong basin were formed in the Early Cretaceous from about 135 Ma to 127 Ma, lasting 8–10 Ma. There were no Jurassic volcanic activities in all the volcanic basins including the Lu-Zong basin in the middle and lower reaches of the Yangtze River area. This work has provided new chronological results for the further study and understanding of the tectonic, magmatic and metallogenic processes of eastern China in the Mesozoic.