Regional Magmatic Event Research Articles

IN SITU U-Pb DATING OF GARNET AND CASSITERITE FROM THE KANBAUK W-Sn(-F) SKARN DEPOSIT, DAWEI REGION, SOUTHERN MYANMAR: NEW INSIGHTS ON THE REGIONAL Sn-W METALLOGENY IN THE SOUTHEAST ASIAN TIN BELT

Abstract Skarn ores have recently been identified beneath the historically mined placer Sn deposit at Kanbauk of the Dawei region, southern Myanmar. A large-tonnage skarn ore reserve at Kanbauk is estimated to be over 100 million tonnes, with reported ore grades of 0.17% WO3, 0.26% Sn, and 15.4% CaF2, potentially making it one of the largest W-Sn skarn deposits in the Southeast Asian tin belt. The mineralized skarns lie between marbles to the east and metasediments of the Mergui Group to the west. The timing of the mineralization is unclear, and thus the genetic relationship with regional magmatic events is not known. We report laser ablation-inductively coupled plasma-mass spectrometry U-Pb ages of garnet and cassiterite from the mineralized skarns. Garnet grains from the massive prograde skarns are typically subhedral to euhedral and show both sector and oscillatory zoning. They have 15 to 23% andradite (Ad), 55 to 67% grossularite (Gr), and 16 to 30% pyralspite (Py) (Ad15-23Gr55-67Py16-30) and contain 0.08 to 306 ppm U with a lower intercept 206Pb/238U age of 56.0 ± 1.5 Ma. Cassiterite grains from retrograde veinlets are subhedral to anhedral and have U contents from 110 to 12,000 ppm with a lower intercept 206Pb/238U age of 54.2 ± 1.7 Ma. Garnet and cassiterite have ages consistent within error and can be taken to indicate the formation of the Kanbauk W-Sn(-F) skarn deposit at around 55 Ma. Together with published ages of primary Sn-W deposits in the Dawei region, our study confirms a westwardly younging trend of mineralization toward the coast and provides support for rollback of the Neo-Tethyan subducting slab since the Late Cretaceous, which is considered as the main mechanism for the regional, extensive Sn-W mineralization.

From Tethyan subduction to Arabia-Eurasia continental collision: Multiple geo-thermochronological signals from granitoids in NW Iran

NW Iran, situated between the Arabian and Eurasian plates, carries a record of both Paleo-Tethyan and Neo-Tethyan tectonic evolution. During the Wilson Cycle of Tethys Ocean opening and closing, several episodes of magmatism from Late Paleozoic to latest Cenozoic generated a massive volume of intrusive rocks. These intrusives, which record cooling histories from high-temperatures to final exhumation, are ideal for multiple geo-thermochronological studies. Our new zircon and apatite U-Pb results suggest three regional magmatic events in the Late Carboniferous, mid-Cretaceous and middle Eocene, which could be related to Paleo-Tethys subduction, Neo-Tethys subduction and Neo-Tethyan ridge subduction, respectively. New apatite fission track and (U-Th)/He data reveal post-magmatic cooling and differential exhumation related to subduction and collision. By integrating published regional thermochronological data from northeastern part of the Middle East, a broader tectono-thermal framework is outlined as follows: 1) Cretaceous cooling signals are most pronounced in the Alborz and Caucasus, and reflect back-arc extension during Neo-Tethyan subduction; 2) diachronous Neo-Tethyan evolution led to the Paleocene assemblage of Anatolia in the west, and subduction associated with a late Paleocene-Eocene magmatic flare-up in Iran. Subsequent westward escape of Anatolia and extensive surface uplift were driven by propagation of Arabia-Eurasia collision in the Miocene; 3) Continuous northward indentation of Arabia into Eurasia triggered late Miocene-Pliocene fast exhumation of the Zagros, Alborz and the Caucasus.

UNRAVELING MINERALIZATION AND MULTISTAGE HYDROTHERMAL OVERPRINTING HISTORIES BY INTEGRATED IN SITU U-Pb AND Sm-Nd ISOTOPES IN A PALEOPROTEROZOIC BRECCIA-HOSTED IOCG DEPOSIT, SW CHINA

Abstract Precambrian iron oxide copper-gold (IOCG) deposits are generally encountered with multistage hydrothermal overprints and hence have complex isotopic records. Precise dating of ore-forming and overprinting events and assessment of time-resolved metal sources are fundamental for understanding ore genesis. Here, we quantify the evolution history by integrating in situ U-Pb dating of texturally constrained allanite and Sm-Nd isotope data of ores and major rare earth element (REE) minerals in the breccia-hosted Lanniping Fe-Cu deposit in Kangdian region, southwestern China. The economically mineralized breccia in Lanniping Fe-Cu deposit is characterized by pervasive and texturally destructive replacement of polymictic clasts, including host metasedimentary packages, the intruded dolerite, and pre-ore halokinetic breccia. Ore minerals in cements are mainly composed of magnetite, chalcopyrite, bornite, and variable amounts of REE-rich minerals (e.g., apatite and allanite/epidote). Two types of allanite were identified in ores. Type I prismatic allanite texturally intergrown with magnetite has a SHRIMP U-Pb age of 1728 ± 20 Ma (1σ), which matches a zircon U-Pb age of 1713 ± 14 Ma (2σ) for the dolerite clasts and provides the direct age constraint on the Fe-Cu mineralization event. Type II anhedral allanite shows complex zoning and is spatially associated with, but texturally later than, magnetite, apatite, and chalcopyrite. This type of allanite yields significantly younger SHRIMP dates of 1015 ± 33 (1σ) and 800 ± 16 Ma (1σ) for cores and rims, respectively, which correspond to discrete regional magmatic events and hence record hydrothermal overprint/remobilization events of ore minerals in the deposit. Integrated Sm-Nd isotope compositions of type I allanite, apatite, and whole ores generally align along the reference Sm-Nd isochron of 1728 Ma, further confirming the primary ore formation at ~1.7 Ga. Corresponding εNd(1728 Ma) values ranging from –2.8 to 0.3 are significantly higher than those of the host metasedimentary rocks (–9.5 to –6.2) but comparable to those of contemporaneous igneous intrusions (–0.3 to 5.3) in the region, demonstrating that REE components of the primary ores were dominantly sourced from rocks of mantle-derived affinity. Both cores and rims of the younger type II allanite grains have Nd isotope compositions consistent with the unique time-evolved line of the ~1.7 Ga ores, implying that REEs incorporated into type II allanite were ultimately sourced from the primary ores in this deposit. The combined texture, chemical, U-Pb, and Sm-Nd isotope data thus demonstrate that REE remobilization was localized during post-ore hydrothermal overprint with negligible external inputs of REEs to the primary ores in the Lanniping deposit. In this contribution, we not only date primary ore formation but also recognize several younger allanite generations that record internal metal redistributions in response to post-ore tectonothermal events. Our study highlights the potential of ore-associated REE minerals such as allanite for resolving the age of multiple stages of hydrothermal events in complex ore deposits by ion probe, provided that careful examination of textural and paragenetic relationship of ores is conducted. Our finding of these younger allanite generations also exemplifies the significance of evaluation on time-resolved metal input for better characterizing the evolution history of the IOCG deposits.

Paleoarchean to Paleoproterozoic crustal evolution in the Guanambi-Correntina block (GCB), north São Francisco Craton, Brazil, unraveled by U-Pb Geochronology, Nd-Sr isotopes and geochemical constraints

The north-central part of the São Francisco Craton (SFC) is one of the oldest terranes in the South American platform and experienced three regional magmatic events at 3.4–3.3, 3.1–2.7, and 2.1 Ga. Our new SHRIMP U-Pb data place the origin of Guanambi-Correntina block (GCB) in the Paleoarchean, contemporaneous with the Gavião block in central north SFC. Lithological, geochemical, and isotopic evidence indicate that the 3.4–3.3 Ga rocks of the Favelândia complex are derived from partial melting of E-MORB sources during flat subduction. The 3.1–2.7 Ga K-granitoids in the Santa Izabel complex are derived from a mixture of crustal and mafic sources, as evidenced by the slightly negative to positive ƐNd(T) and Sr/Sri between 0.700 and 0.706. The 3.1–2.7 Ga event is responsible for the recycling of the Paleoarchean TTG crust, generation of low- to high-K evolved crust, and juxtaposition of the Favelândia and Santa Izabel complexes. The youngest magmatic event at ca. 2.1 Ga produced felsic plutons, derived from anatexis of Archean felsic rocks and mantle components. The collisional process tectonically juxtaposed the Riacho de Santana greenstone belt and Archean felsic rocks at 2.1–2.0 Ga, determining the region’s current configuration. Based on the data reported in this study and previously published data, we suggest that the Guanambi-Correntina block was formed as an independent unit in the northern SFC.

Recognition of Late Triassic Cu-Mo Mineralization in the Northern Yidun Arc (S.E. Tibetan Plateau): Implications for Regional Exploration

The Yidun arc, located in the southeastern Tibetan Plateau, was formed by the westward subduction of the Ganze-Litang Paleo-Tethys ocean in Late Triassic. It is well-known for the formation of numerous Mesozoic porphyry-skarn Cu-Mo-(Au) deposits in the arc. To date, more than 20 Cu-Mo-(Au) deposits (>10 million tonnes Cu resources) have been discovered in the southern Eastern Yidun arc. However, few Cu-Mo deposits have been discovered in the northern Eastern Yidun arc. In recent years, some Cu-Mo deposits or occurrence are successively discovered in the northern Eastern Yidun arc, but their ore-forming ages are not well constrained. It remains unclear whether such Cu-Mo mineralization formed by similar metallogenic event and geodynamic setting as the Cu-Mo-(Au) mineralization in the south. In order to determine the metallogenic age and shed light on potential links between Cu-Mo mineralization and regional magmatic events, we present molybdenite Re-Os and zircon U-Pb ages to constrain the timing of two types of Cu-Mo mineralization in the northern Eastern Yidun arc (type I and type II). Molybdenite ICP-MS Re-Os dating results show that type I mineralization was formed at 217.7 ± 3.6 Ma, which is highly consistent with the formation ages of the host granite (218.1 ± 1.5 Ma, 2σ, n = 15, MSWD = 0.92) and aplite dyke (217.3 ± 1.3 Ma, 2σ, n = 16, MSWD = 0.50) within error. While the type II mineralization has a relatively younger formation age of 211.8 ± 4.7 Ma than the host granite (217.1 ± 1.5 Ma, 2σ, n = 14, MSWD = 0.96) and type I Cu-Mo mineralization. These data indicate that the Cu-Mo mineralization in the northern Eastern Yidun arc was temporally and spatially related to the Late Triassic magmatism in the region. Rhenium (Re) concentrations in the molybdenite from type I mineralization, ranging from 12.77 to 111.1 ppm (typically > 100 ppm), indicate that the ore-forming metals were derived mainly from a mantle source. However, Re contents in molybdenite from the type II mineralization, ranging from 7.983 to 10.40 ppm, indicate that the ore-forming metals were derived from a mixed mantle and crustal source with a predominantly crustal component. This study confirms that the northern Eastern Yidun arc exists Late Triassic Cu-Mo metallogenesis, and thus much attention should be paid on this region to find more Late Triassic Cu-Mo resources.

Rb–Sr geochronology and geochemistry of pyrite from the Shihu gold deposit, central North China Craton: Implication for the timing and genesis of gold mineralization

The Shihu quartz vein‐type gold deposit in the central North China Craton occurs within Neoarchean TTG (tonalite‐trondhjemite‐granodiorite) gneisses and shows spatial relationship with quartz diorite porphyry. The occurrence of the ore body is controlled by the N‐S fracture. Primary mineral assemblage of the ore body is composed of quartz, calcite, and base‐metal sulphides, including pyrite, galena, sphalerite, chalcopyrite, and pyrrhotite. Eight pyrite samples from veins of the main mineralization stage yield an isochron age of 129 ± 3 Ma (MSWD = 1.6), coinciding with the major Early Cretaceous magmatic event, and associated lithospheric thinning recognized in the NCC. The (87Sr/86Sr)i values of the pyrite (0.7102 ± 0.0006) are lower than the continental crust mean value (0.719) and higher than that of the mantle initial ratio (0.707), suggesting mixed crust–mantle source for the metallogenic components. Three types of hydrothermal pyrite from the early to major ore‐forming stages were identified in the quartz vein, including Py(I) as disseminated euhedral grains, Py (II) as dense bulks, and Py (III) as intergrowth with base‐metal sulphides. Electron microprobe analysis of the pyrite grains indicates a trend from S‐deficient to S‐enriched and from Fe‐enriched to Fe‐deficient stages, suggesting an increase in sulphur fugacity and reducing conditions. The arsenic in pyrite shows decrease against increasing Au during Stages II and III, implying that As is not the only trigger of gold precipitation, but electrochemical‐geochemical barrier played a major role in the formation of the ore. The Co and Ni concentrations, as well as Co/Ni ratios in pyrite, are relatively high, which provide an additional clue to the link with the regional magmatic event. Combined with the textural features, we propose a wider potential for gold exploration in this region, linking the mineralization with a major transitional tectonic regime.

Discovery of Middle–Late Devonian and Early Permian magmatic events in East Asia and their implication for the Indosinian orogeny in South China: Insights from the sedimentary record

AbstractWhether the driver of the Indosinian orogeny in the South China block was related to the evolution of the Paleotethyan Ocean or the Paleo-Pacific Ocean has been a point of much debate. We applied detrital zircon U-Pb dating to Permian–Triassic sedimentary rocks from South China to trace sediment provenance and to further test these models. Our results, combined with other published data from the Pingxiang, Youjiang, Yong’an, and Yongding Basins, show that 400–350 Ma and 300–260 Ma zircon grains are ubiquitous throughout the entirety of southern South China. This indicates regional magmatic events as potential sources. The discovery of Middle–Late Devonian and Early Permian igneous rocks, tuffs, and volcaniclastic rocks in Southeast Asia and Hainan Island implies the presence of two magmatic events (400–350 Ma and 300–260 Ma) within or beyond the southern margin of South China. This information, together with the mostly negative εHf(t) values of 400–350 Ma and 300–260 Ma zircon grains, arc-like geochemical signatures of the possible source rocks, and the regional geology of East Asia, suggests that they originated from sources related to Paleotethyan and even Proto-Tethyan subduction. Thus, Permian–Triassic sedimentation and the Indosinian orogeny in South China were largely controlled by the evolution of the Tethyan Ocean.

Geochemistry of I- and A-type granites of the Qingyang–Jiuhuashan complex, eastern China: Insights into early cretaceous multistage magmatism

The Lower Yangtze River Belt in eastern China preserves evidence for widespread Mesozoic magmatism. Geochronological and geochemical studies have been conducted on the magmatic rocks and their associated mineral deposits; however, the relationships and melting conditions of spatially associated I- and A-type granites remain poorly constrained. The Qingyang–Jiuhuashan granitic complex in southern Anhui Province, eastern China, comprises primarily hornblende-bearing granodiorite, monzogranite, alkali-feldspar granite, and granitic porphyry, which yield zircon UPb ages of 142.2 ± 1.3, 134.6 ± 1.2, 126.4 ± 1.9, and 125.9 ± 1.2 Ma, respectively. Zircon grains from an enclave in the monzogranite yield an age of 134.5 ± 1.1 Ma, similar to that of the host rock. The ages are consistent with the timing of regional magmatic events in southern Anhui Province. Whole-rock geochemical compositions indicate that the granodiorite and monzogranite represent I-type granites, whereas the alkali-feldspar granite and granitic porphyry display A-type signatures. The I- and A-type granites yield similar whole-rock SrNd isotopic compositions, but distinct zircon δ18O values. We infer that the I-type granites were produced through water-fluxed melting of Mesoproterozoic basement during subduction of the paleo-Pacific plate. The A-type granites were generated through partial melting of the lower crust, triggered by extensional upwelling of mantle-derived magmas that provided heat and alkali-rich fluids. We identify three stages of Cretaceous tectono-magmatic activity that occurred in response to ridge subduction and subsequent slab rollback beneath eastern China.

Timing of the final closure of the Irtysh–Zaysan Ocean: New insights from the earliest stitching pluton in the northern West Junggar, NW China

A tight constraint on the closure time of the Irtysh–Zaysan Ocean, a major southwestern branch of the Paleo‐Asian Ocean, is crucial for understanding the final amalgamation of the Central Asian Orogenic Belt; however, no agreement has been reached. This study presents new zircon U–Pb ages and whole‐rock geochemical data for the Keluke composite pluton in the Zharma–Saur arc of the northern West Junggar (NW China). The Keluke composite pluton mainly includes the hornblende gabbro, gabbroic diorite, and K‐feldspar granite. The laser ablation inductively coupled plasma mass spectrometry zircon U–Pb dating indicates that the hornblende gabbro and K‐feldspar granite formed at 334 ± 1 Ma and 319 ± 4 Ma, respectively. The gabbroic and dioritic rocks are calc‐alkaline and arc‐related plutonism characterized by strong depletions in Nb–Ta–Ti, thus being the product of the subduction of the Irtysh–Zaysan Ocean. By contrast, the K‐feldspar granite is geochemically similar to the post‐collisional highly fractionated I‐type granite. In particular, when compared with regional magmatic events, the Keluke K‐feldspar granite represents the first stitching plutonism in response to the termination of the Irtysh–Zaisan Ocean, which strongly indicates that the Irtysh–Zaisan Ocean was closed prior to the earliest late Carboniferous.

Geochronology of the Limu W–Sn–Nb–Ta‐Bearing Granite Pluton in South China

AbstractLimu W–Sn–Nb–Ta mining district is located in the Nanling Range W–Sn poly‐metallic mineralization belt in south China. The district includes a number of Sn–Nb–Ta and W–Sn ore occurrences; all of them are spatially associated with granite stocks of a largely‐unexposed pluton, the Limu granitic pluton. A granite sample collected from the Sn–Nb–Ta‐bearing Jinzhuyuan granite stock yields a zircon SHRIMP U–Pb age of 218.3 ± 2.4 Ma, a muscovite 40Ar/39Ar plateau age of 212.4 ± 1.4 Ma, and a muscovite 40Ar/39Ar isochron age of 213.2 ± 2.2 Ma. Another granite sample collected from the W–Sn‐bearing Sangehuangniu granite stock yields a zircon SHRIMP U–Pb age of 214 ± 5 Ma. The geochronological data provide new constraints on the age of the Limu granite pluton and the timing of the associated W–Sn–Nb–Ta mineralization—at least it sets a reasonable upper age limit for the mineralization of the W–Sn–Nb–Ta ores. The reported ages suggest an active Late Triassic granitic magmatism in Limu area which is part of a regional magmatic event near the end of the Indosinian orogeny in south China.

The geologic features of mineralization at the Dongsheng uranium deposit in the northern Ordos Basin ( Central China)

The Dongsheng uranium deposit, localized in the northern Ordos Basin in northern Central China, is among the largest deposits of thistype in China. It occurs in the sandstones of the Middle Jurassic Zhiluo Formation. The crystalline rocks of the Yinshan and Daqingshanorogenic belts in the northern Ordos Basin are the source of uranium for the deposit. The Dongsheng deposit shows the main characteristicsof a sandstone uranium deposit of interstratal oxidation zone, including specific hydrothermal rock alteration. It displays distinct lithologicand mineral zoning. The sandstones of the oxidation zone are variegated and oxidized, and the sandstones of the reduction zone are gray andlight gray medium- to coarse-grained and underwent albitization, carbonatization, chloritization, epidotization, pyritization, etc. Ore-bearingsandstones occur in the transitional oxidation-reduction zone, being confined mainly to the reduction zone. Uranium minerals are predominanthigh-temperature coffinite and subordinate brannerite and uraninite. The homogenization temperature of fluid inclusion in the sandstone cementis 150–160 °C. The performed geological analysis suggests that the hydrothermal alteration of the uranium deposit is intimately related to thesynchronous Mesozoic regional magmatic events in the Ordos Basin.

SHRIMP-RG U-Pb isotopic systematics of zircon from the Angel Lake orthogneiss, East Humboldt Range, Nevada: Is this really Archean crust? REPLY

New SHRIMP-RG (sensitive high-resolution ion microprobe–reverse geometry) data confirm the existence of Archean components within zircon grains of a sample from the orthogneiss of Angel Lake, Nevada, United States, previously interpreted as a nappe of Archean crust. However, the combined evidence strongly suggests that this orthogneiss is a highly deformed, Late Cretaceous monzogranite derived from melting of a sedimentary source dominated by Archean detritus. Zircon grains from the same sample used previously for isotope dilution–thermal ionization mass spectrometry (ID-TIMS) isotopic work were analyzed using the SHRIMP-RG to better define the age and origin of the orthogneiss. Prior to analysis, imaging revealed a morphological variability and intragrain, polyphase nature of the zircon population. The SHRIMP-RG yielded 207Pb/206Pb ages between ca. 2430 and 2580 Ma (a best-fit mean 207Pb/206Pb age of 2531 ± 19 Ma; 95% confidence) from mostly rounded to subrounded zircons and zircon components (cores). In addition, several analyses from rounded to subrounded cores or grains yielded discordant 207Pb/206Pb ages between ca. 1460 and ca. 2170 Ma, consistent with known regional magmatic events. All cores of Proterozoic to latest Archean age were encased within clear, typically low Th/U (<0.015), oscillatory zoned, mostly euhedral, Late Cretaceous zircon. The younger zircon yielded essentially concordant 206Pb/238U ages between 72 and 91 Ma, consistent with magmatic ages from Lamoille Canyon to the south. An age of ca. 90 Ma is suggested, the younger 206Pb/238U ages resulting from Pb loss. The Cretaceous and Precambrian zircon components also have distinct trace element characteristics, indicating that these age groups are not related to the same igneous source. These results support recent geophysical interpretations and negate the contention that the Archean-Proterozoic boundary extends into the central Great Basin area. They further suggest that the world-class gold deposits along the Carlin Trend are not underlain by Archean cratonal crust, but rather by the Proterozoic Mojave province and Neoproterozoic and Paleozoic metasedimentary sequences dominated by detritus derived from Late Archean sources rather than Proterozoic sources, as is evident farther to the south in the Ruby Mountains.

SHRIMP-RG U-Pb isotopic systematics of zircon from the Angel Lake orthogneiss, East Humboldt Range, Nevada: Is this really Archean crust

New SHRIMP-RG (sensitive high-resolution ion microprobe–reverse geometry) data confirm the existence of Archean components within zircon grains of a sample from the orthogneiss of Angel Lake, Nevada, United States, previously interpreted as a nappe of Archean crust. However, the combined evidence strongly suggests that this orthogneiss is a highly deformed, Late Cretaceous monzogranite derived from melting of a sedimentary source dominated by Archean detritus. Zircon grains from the same sample used previously for isotope dilution–thermal ionization mass spectrometry (ID-TIMS) isotopic work were analyzed using the SHRIMP-RG to better define the age and origin of the orthogneiss. Prior to analysis, imaging revealed a morphological variability and intragrain, polyphase nature of the zircon population. The SHRIMP-RG yielded 207Pb/206Pb ages between ca. 2430 and 2580 Ma (a best-fit mean 207Pb/206Pb age of 2531 ± 19 Ma; 95% confidence) from mostly rounded to subrounded zircons and zircon components (cores). In addition, several analyses from rounded to subrounded cores or grains yielded discordant 207Pb/206Pb ages between ca. 1460 and ca. 2170 Ma, consistent with known regional magmatic events. All cores of Proterozoic to latest Archean age were encased within clear, typically low Th/U (<0.015), oscillatory zoned, mostly euhedral, Late Cretaceous zircon. The younger zircon yielded essentially concordant 206Pb/238U ages between 72 and 91 Ma, consistent with magmatic ages from Lamoille Canyon to the south. An age of ca. 90 Ma is suggested, the younger 206Pb/238U ages resulting from Pb loss. The Cretaceous and Precambrian zircon components also have distinct trace element characteristics, indicating that these age groups are not related to the same igneous source. These results support recent geophysical interpretations and negate the contention that the Archean-Proterozoic boundary extends into the central Great Basin area. They further suggest that the world-class gold deposits along the Carlin Trend are not underlain by Archean cratonal crust, but rather by the Proterozoic Mojave province and Neoproterozoic and Paleozoic metasedimentary sequences dominated by detritus derived from Late Archean sources rather than Proterozoic sources, as is evident farther to the south in the Ruby Mountains.

Tectonic correlations of pre-Mesozoic crust from the northern termination of the Colombian Andes, Caribbean region

Reconnaissance zircon U/Pb SHRIMP, Ar–Ar, and Sm–Nd geochronology, petrological, and geochemical data were obtained from selected localities of two pre-Mesozoic metamorphic belts from the northern termination of the Colombian Andes in the Caribbean region. The older Proterozoic belt, with protoliths formed in a rift- or backarc-related environment, was metamorphosed at 6–8 kb and 760–810 °C during Late Mesoproterozoic times. This belt correlates with other high-grade metamorphic domains of the Andean realm that formed a Grenvillian-related collisional belt linked to the formation of Rodinia. The younger belt was formed over a continental arc at <530–450 Ma in a Gondwanide position and metamorphosed at 5–8 kb and 500–550 °C, probably during the Late Paleozoic–Triassic, as part of the terranes that docked with northwestern South America during the formation of Pangea. A Mesozoic Ar–Ar tectonothermal evolution can be related to regional magmatic events, whereas Late Cretaceous–Paleocene structural trends are related to the accretion of the allocthonous Caribbean subduction metamorphic belts. Lithotectonic correlations with other circum-Caribbean and southern North American pre-Jurassic domains show the existence of different terrane dispersal patterns that can be related to Pangea’s breakup and Caribbean tectonics.

The East Bull Lake Intrusive Suite: Remnants of a 2.48 GaLarge Igneous and Metallogenic Province in the Sudbury Area of the CanadianShield

The 2.49 to 2.475 Ga intrusions of the East Bull Lake intrusive suite occur in an east-northeast-trending discontinuous belt along the presently exposed boundary between the Archean Superior and the Proterozoic Southern provinces of the Canadian Shield near Sudbury, Ontario. The East Bull Lake intrusive suite is part of a regional Paleoproterozoic magmatic event, extending from 2.49 to 2.44 Ga, which also includes bimodal volcanic rocks, felsic plutons, and the regionally extensive Hearst and Matachewan dike swarms. This regional magmatic event is thought to be the result of a mantle plume-driven, intracontinental rifting event that led to the development of a major basin to the south that was subsequently filled by sedimentary rocks of the Huronian Supergroup. The East Bull Lake intrusive suite appears to have been emplaced along a major axial-rift fault related to this rifting event. Compelling field and geochemical evidence summarized here indicates that the three largest East Bull Lake suite intrusions, the East Bull Lake, Agnew Lake, and River Valley intrusions, crystallized from similar, low Ti, high Al tholeiitic parent magmas that originated in deeper, more basic chambers. It is proposed that the primary magmas for the East Bull Lake suite intrusions were second-stage melts derived from a sublithospheric depleted mantle source that had been modified by Neoarchean subduction and accretion events. Most of the petrological characteristics of the East Bull Lake suite intrusions reflect plagioclase-dominated fractional crystallization that generated a pronounced Fe enrichment trend in the residual magmas. Orthopyroxene is the high-temperature pyroxene and occurs as a cumulus phase through most of the stratigraphy of each intrusion of the suite. Local olivine-rich cumulates, present in all of the major East Bull Lake suite plutons, in some instances appear to be comagmatic with associated plagioclase-rich cumulates. A distinctive marginal facies, comprising brecciated and locally thermally recrystallized and/or partially melted footwall rocks, is a common feature of the East Bull Lake suite plutons and indicates that a high-energy flow regime existed during the initial stages of emplacement. This marginal unit typically grades into a heterolithic, inclusion-rich gabbronorite that contains erratically distributed mafic autoliths and footwall-derived xenoliths. Chemical contamination of the mineralized matrix of these inclusion- bearing rocks can be extreme at a local scale. The inclusion-rich zone is overlain by a thick interval of undifferentiated, plagioclase-rich cumulates that locally display spectacular glomerocrystic textures. Rhythmically and irregularly, modally layered leucogabbronorite and gabbronorite make up much of the middle parts of the stratigraphy. Massive to crudely layered ferrogabbro and ferrosyenite only occur in the uppermost part of the Agnew Lake intrusion. Varitextured gabbro and pegmatitic gabbro occur as irregular layers and local veins and pods throughout the stratigraphy. The East Bull Lake suite is currently being explored for contact-type platinum-group element (PGE)-Cu-Ni mineralization that has a clear spatial association with orthopyroxene-rich cumulates which are otherwise poorly represented in these intrusions. Disseminated PGE- and Cu-rich sulfide mineralization is concentrated within the inclusion-bearing zones but extends upward into overlying plagioclase cumulates and appears to have collected along the lower margins of these intrusions through the combined action of vigorous convection of the resident magma and downward percolation of dense, mafic residual liquid. The resultant narrow zones of sulfide mineralization were thus derived from a much larger volume of parent magma. Feeder dikes to the mineralized parts of the intrusions appear to have been saturated in sulfide upon emplacement and had relatively high background PGE contents compared to those for other known magmatic PGE deposits; however, the economic potential of the contact-type mineralization hinges on the efficacy of the physical concentration process, which upgraded the metal content and size of the original sulfide particles inside the magma chambers. Based on the proposed basal accumulation model, the best prospective mineralization is most likely to occur in embayments along the sidewall and floor of the intrusions.

Continental margin off Norway 62–75°N: Palaeogene tectono-magmatic segmentation and sedimentation

Abstract The development of rifted and sheared segments on the Norwegian continental margin between 62 and 75°N is spatially related to a distinct along-margin segmentation which is governed by transfer zones formed during the Late Jurassic-Early Cretaceous rift episode. In fact, these structures may represent a repeated structural inheritance going back to the Proterozoic. Complete lithospheric breakup near the Paleocene-Eocene transition was preceded by a rift episode which was probably initiated in the early to middle Campanian. It culminated with a massive, regional magmatic event during breakup characterized by eruption of thick lava sequences covering large areas along the continent-ocean transition. The Norwegian volcanic margin belongs to the North Atlantic Large Igneous Province formed by impingement of the Iceland plume on a lithosphere under extension. Offsets in the initial plate boundary, combined with late rift uplift and subsequent construction of emerged marginal highs during breakup, provide key constraints on the Palaeogene water mass circulation, basin evolution and sedimentation during a period of progressive environmental deterioration following early Eocene greenhouse conditions.

A Mid Cretaceous origin for the Galápagos hotspot: volcanological, petrological and geochemical evidence from Costa Rican oceanic crustal segments

The Quepos, Nicoya and Herradura oceanic igneous terranes in Costa Rica are conspicuous features of a Mid to Late Cretaceous regional magmatic event that encompasses similar terranes in Central America, Colombia, Ecuador and the Caribbean. The Quepos terrane (66 Ma), which consists of ol-cpx phyric, tholeiitic pillow lavas overlain by highly vesicular hyaloclastites, breccias and conglomerates, is interpreted as an uplifted seamount/ocean island complex. The Nicoya (∼90 Ma) and Herradura terranes consist of fault-bounded sequences of sediments, tholeiitic volcanics (pillow lavas and massive sheet flows) and plutonic rocks. The volcanic rocks were emplaced at relatively high eruption rates in moderate to deep water, possibly forming part of an oceanic plateau. Major and trace element data from Nicoya/Herradura tholeiites indicate higher melting temperatures than inferred for normal mid-ocean-ridge basalts (MORB) and/or a different source composition. Sr–Nd–Pb isotopic ratios from all three terranes are distinct from MORB but resemble those from the Galapagos hotspot. The volcanological, petrological and geochemical data from Costa Rican volcanic terranes, combined with published age data, paleomagnetic results and plate tectonic reconstructions of this region, provide strong evidence for a Mid Cretaceous (∼90Ma) age for the Galapagos hotspot, making it one of the oldest known, active hotspots on Earth. Our results also support an origin of the Caribbean Plate through melting of the head of the Galapagos starting plume.

Winagami reflection sequence: Seismic evidence for postcollisional magmatism in the Proterozoic of western Canada

A recent Lithoprobe seismic-reflection transect in the Peace River arch region of the Western Canada sedimentary basin traverses continental crust that was assembled in a series of magmatic and collisional events during the interval 2.0–1.9 Ga. The seismic data reveal panels of dipping reflectivity at tectonic domain boundaries, but they are otherwise dominated by prominent, subhorizontal reflections that occur between 3.5 and 18.5 km depth over an area of at least 120 000 km 2 . These reflections, here named the Winagami reflection sequence, overlap several tectonic domains and are comparable in geometry, depth, and areal distribution to sills and discordant intrusive sheets imaged by previous seismic profiling in the Baltic shield and the Basin and Range province of the western United States. As many as five distinct reflections are present, some of which can be traced for more than 100 km. Contrasting reflection character in adjacent tectonic domains suggests an intrusive geometry controlled partly by the mechanical properties of the host rocks. Seismic modeling indicates that observed waveforms are consistent with reflections from tabular mafic bodies ∼70 m thick. The Winagami reflection sequence crosscuts the collisional fabric (ca. 1890 Ma), but the sequence is offset by a brittle fault with an estimated age of 1760 Ma. These timing constraints suggest that the Winagami reflection sequence is a previously unknown component of a regional magmatic event emplaced during a period of brittle indentation of the western Canadian shield by the Slave province.

The Shabogamo Intrusive Suite, Labrador: Sr and Nd isotopic evidence for contaminated mafic magmas in the Proterozoic

The Shabogamo Intrusive Suite comprises numerous bodies of variably metamorphosed gabbro which intrude Archean and Proterozoic sequences at the junction of the Superior, Churchill, and Grenville structural provinces in western Labrador. Combined Sm-Nd and Rb-Sr systematics in two bodies, ranging from unmetamorphosed to lightly metamorphosed, document a crystallization age of about 1375 m.y., and suggest that both bodies crystallized from magmas with similar Nd and Sr isotopic compositions. This age is in accordance with the existence of a regional magmatic event in the Churchill Province at approximately 1400 m.y. Rb-Sr systematics in two bodies of amphibolite-grade gabbro suggest a regional metamorphic event at about 950 m.y., corresponding to the waning stages of Grenville activity. Sm-Nd systematics in these high-grade bodies are affected to a much lesser degree than Rb-Sr. Initial ratios for 143Nd/ 144Nd and 87Sr/ 86Sr are lower and higher, respectively, than bulk earth values at 1375 m.y. Both these displacements are in the direction of older crustal material at 1375 m.y., and a model is proposed to produce the Shabogamo magma by mixing a mantle-derived magma with a partial melt of crustal rocks (approximately 4: 1 by volume). Young volcanic rocks with anomalous Nd and Sr isotopic ratios, which have previously been taken as evidence for “enriched” mantle, may be interpreted similarly.