Ages Of Magmatic Rocks Research Articles

Geochronology and petrogenesis of Early Mesozoic Rb-mineralized granitic rocks from the Liuzhangshan Rb, West Qinling Orogen Belt, China

ABSTRACT The newly discovered Liuzhangshan (LZS) Rb deposit is a large granite-type deposit in the West Qinling Orogenic Belt (WQOB), spatially associated with biotite monzogranite. Previous research has focused on the geological characteristics and metallogenic conditions of the LZS Rb deposit; however, its geochronology and petrogenesis remain unclear. In this study, monazite U-Pb dating of biotite monzogranite in the LZS deposit yielded a U-Pb age of 200.8 ± 0.9 Ma via laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Combined with the age of magmatic rocks in the WQOB, we propose a potential mineralization event related to Rb around 200 Ma in the WQOB. The biotite monzogranite belongs to weakly peraluminous, high-K calc-alkaline serious, containing high SiO2, Na2O, K2O, and low FeO, MgO, CaO, and P2O5. Compared with primitive mantle and relatively unfractionated granodiorites found in the WQOB, the biotite monzogranite enriched in Rb, Th, U, and Pb, deleted in Ba, Sr, and Ti, and showed negative Eu anomalies, a high Rb/Sr ratio, and a low K/Rb ratio, an ∑REE of 31.3 ppm, an LREE/HREE ratio of 6.48, a LaN/YbN ratio of 2.59, an Mg# of 4.15, and a crystallization temperature of 760°C, the biotite monzogranite also had a high differentiation index (DI = 98.2), apparent tetrad effect (TE 1,3 = 1.12 ~ 1.15), characteristic of highly fractionated I-type granite. Monzoite from the biotite monzogranite in the LZS deposit has negative εNd(t) values ranging from −12.45 to −11.17, and TDM1 of 1569 ~ 2018 Ma, indicating that the LZS biotite monzogranite originated from the partial melting of a Mesoproterozoic ancient basement material, with a minor contribution from mantle-derived melts. Highly differentiated, low Sr and Yb contents, Sr/Yb ratios, and low crystallization temperature magma facilitated the enrichment of Rb.

Petrogenesis of Variscan and Yanshanian magmatic rocks in the Longjiang region of Heilongjiang province and implications for ore exploration

The Longjiang region is located on the eastern slope of the southern section of the Great Xing'an Range, in the transition zone between the Songliao Basin and Great Xing'an Range uplift. As it is an extension of the Inner Mongolia Linxi-Tuquan Cu-Mo-Pb-Zn metallogenic belt in the Heilongjiang region, it has conditions conducive to the formation of porphyry deposits. We report zircon U-Pb dating and whole-rock geochemistry data of magmatic rocks in representative mineralized points in the Longjiang region. Zircon U-Pb age shows that the age of magmatic rocks is concentrated in the Variscan period (312 ± 5 ∼ 294 ± 2Ma) and Yanshanian period (134 ± 2 ∼ 123 ± 1Ma). According to whole-rock geochemical analysis, the granites have SiO2 contents of 61.2–69.16 % (66.9 % on average), Al2O3 contents of 13.87–15.98 %, MgO contents of 0.6–2.22 %, TiO2 contents of 0.25–0.88 %, K2O contents of 2.70–3.63 %, Na2O/K2O ratios of 1.05–2.05, and high K2O+Na2O contents (5.8–7.17 %), which are consistent with adakitic rocks. Yanshanian granites have A/CNK values of 1.02–2 and an average differentiation index of 84.7, which indicates that they are highly differentiated I-type granites. Variscan I-type granites represent the post-collisional setting of the Paleo-Asian Ocean and are relatively scarce in the region. Yanshanian highly differentiated I-type granites are products of the Paleo-Pacific Ocean's post-collisional retreat and are widely distributed in the region. As the materials and fluids that formed the Liujiushan, Haiyang, and Fendou deposits were derived from magmatic activities, and Variscan and Yanshanian magmas have excellent ore-forming potential. The highly differentiated I-type granites formed by intense Yanshanian magmatism are the most promising candidates for ore exploration in this region.

New U-Pb zircon geochronological data for Takestan magmatic rocks (Western Alborz) and their significance for the interpretation of Paleogene magmatism in Iran

ABSTRACT The Takestan area is in the western part of the Alborz magmatic arc. Magmatic rocks of the area consist of plutonic rocks (e.g. granitoids), effusive volcanic, and pyroclastic rocks. Using U-Pb zircon LA ICP-MS dating, we conclude that major parts of Takestan plutonic rocks were emplaced at 41–39 Ma (Late Eocene, Bartonian), but a small part of these rocks have ages of ~37 Ma (Late Eocene, Priabonian). The dacitic rocks have an age of ~39 Ma (Late Eocene, Bartonian) and the rhyolitic rocks are the youngest part of the magmatic rocks of the region with ages of 37–35 Ma (Late Eocene, Priabonian). Old zircons are present in all of granitoid and volcanic samples, except for a dacitic sample. They are interpreted both as earlier components in a long-lived magma chamber and inherited zircons from older continental crust. The age of magmatic rocks in the western part of the Alborz magmatic arc decreases from east to west, but the ages of the majority of them are limited to Palaeogene. The studied rocks like other Palaeogene magmatic rocks of Iran were possibly formed in a subduction related tectonic environment. Indeed, the Palaeogene magmatism of Iran is akin to geodynamic events related to Neotethyan subduction beneath Iranian micro-continent at the southern part of Eurasia.

The Kultuma Au–Cu–Fe-Skarn Deposit (Eastern Transbaikalia): Magmatism, Zircon Geochemistry, Mineralogy, Age, Formation Conditions and Isotope Geochemical Data

The Kultuma deposit is among the largest and most representative Au–Cu–Fe–skarn deposits situated in Eastern Transbaikalia. However, its genetic classification is still a controversial issue. The deposit is confined to the similarly named massif of the Shakhtama complex, which is composed mainly of quartz monzodiorite-porphyry and second-phase monzodiorite-porphyry. The magmatic rocks are characterized by a low Fe2O3/FeO ratio, low magnetic susceptibility and belong to meta-aluminous, magnesian high-potassic calc-alkalic reduced granitoids of type I. The results of 40Ar-39Ar and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U-Pb dating showed that the formation of magmatic rocks proceeded during the Late Jurassic time: 161.5–156.8 Ma. Relatively low Ce/Ce*, Eu/Eu* and Dy/Yb ratios in the zircons indicate that the studied magmatic rocks were formed under relatively reduced conditions and initially contained a rather low amount of magmatic water. A mineralogical–geochemical investigation allowed us to outline five main stages (prograde skarn, retrograde skarn, potassic alteration, propylitic (hydrosilicate) alteration and late low-temperature alteration) of mineral formation, each of them being characterized by a definite paragenetic mineral association. The major iron, gold and copper ores were formed at the stage of retrograde skarn and potassic alteration, while the formation of polymetallic ores proceeded at the stage of propylitic alteration. The obtained timing of the formation of retrograde skarn (156.3 Ma) and magmatic rocks of the Shakhtama complex, along with the direct geological observations, suggest their spatial–temporal and genetic relationship. The data obtained on the age of magmatic rocks and ore mineralization are interpreted as indicating the formation of the Kultuma deposit that proceeded at the final stages of collision. Results of the investigation of the isotope composition of S in sulfide minerals point to their substantial enrichment with the heavy sulfur isotope (δ34S from 6.6 to 16‰). The only exclusion with anomalous low δ34S values (from 1.4 to 3.7‰) is pyrrhotite from retrograde skarns of the Ochunogda region. These differences are, first of all, due to the composition of the host rocks. Results of the studies of C and O isotope composition allow us to conclude that one of the main sources of carbon was the host rocks of the Bystrinskaya formation, while the changes in the isotope composition of oxygen are mainly connected with decarbonization processes and the interactions of magmatic fluids, host rocks and meteoric waters. The fluids that are responsible for the formation of the mineral associations of retrograde skarns and the zones of potassic alteration at the Kultuma deposit were reduced, moderately hot (~360–440 °C) and high-pressure (estimated pressure is up to 2.4 kbar). The distinguishing features of the fluids in the zones of potassic alteration at the Ochunogda region are a lower concentration and lower estimated pressure values (~1.7 kbar). The propylitic alteration took place with the participation of reduced lower-temperature (~280–320 °C) and lower-pressure (1–1.2 kbar) fluids saturated with carbon dioxide, which were later on diluted with meteoric waters to become more water-rich and low-temperature (~245–260 °C). The studies showed that the main factors that affected the distribution and specificity of mineralization are magmatic, lithological and structural–tectonic ones. Results of the studies allow us to classify the Kultuma deposit as a Au–Cu–Fe–skarn deposit related to reduced intrusion.

Paleogene carbonate systems of Saint Barthélemy, Lesser Antilles: stratigraphy and general organization

Saint Barthelemy is the only island of the northern Lesser Antilles where it is possible to investigate in the detail the chronostratigraphy of the mixed carbonate-volcaniclastic deposits of the Paleogene Caribbean volcanic arc. Based on field mapping and new biostratigraphical and sedimentological data, we study the limestone units interbedded in the Paleogene volcaniclastic deposits. We find that four main carbonate units occur instead of previously believed six ones. Based on the ages given by the foraminifera assemblages, and taking into account the recently published 40Ar/39Ar ages of magmatic rocks, the Lower Limestone unit dates Lutetian, the Intermediate Limestone unit late (?) Bartonian-late Priabonian, the Upper Limestone late Priabonian and the Top Limestone unit early Miocene. The Paleogene carbonate beds were deposited on gently dipping submarine volcaniclastic deposits issued from emergent volcanoes, in muddy, unrimmed inner to mid-ramp setting dominated by bottom communities. A major subaerial unconformity is evidenced during the Oligocene, most probably corresponding to uplift affecting Saint Barthelemy. Our work offers a revised lithostratigraphic succession of Saint Barthelemy as a first key-point for further studies of the Paleogene Caribbean arc deposits which were dismembered following the entrance of the Bahamas Bank in the Lesser Antilles subduction zone.

Monzonitoid magmatism of the copper-porphyritic Lazurnoe deposit (South Primor’e): U-Pb and K-Ar geochronology and peculiarities of ore-bearing magma genesis by the data of isotopic-geochemical studies

Magmatic rocks from the copper-porphyritic Lazurnoe deposit (Central Primor’e) have been studied. It has been found that rocks from the Lazurnyi massif are referred to gabbro-monzodiorites, monzodiorites, and monzo-granodiorites formed during two magmatic phases of different ages. The earlier phase is represented by gabbro-monzodiorites and diorites of the North Stock, and the later one, by gabbro-monzodiorites and monzo-grano-diorites of the South Stock. On the basis of isotopic dating by the U-Pb (SHRIMP) method for zircon and by the K-Ar method for hornblendes and biotites, the age of magmatic rocks is determined at 110 ± 4 for the earlier phase and at 103.5 ± 1.5 for the later one. Examination of the isotopic composition for Nd, Sr, Pb, Hf, δ18O, and REE spectra has shown that melts of the first phase are contaminated with crustal rocks and they are typical for a high degree of secondary alterations. Potassiumfeldspar, biotite, propylitic alterations, and sulfidization are manifested in these rocks. The rocks of the later stage of magmatism are characteristic for a primitive composition of isotopes and the absence of secondary alterations. They carry the features of adakite specifics that allows us to consider them derivatives of mantle generation under high fluid pressure. The intrusion of fluid-saturated melts of the second phase into the magmatic source of the first phase caused both an alteration pattern of rocks and copper-porphyritic mineralization. Isotopes of sulfur and oxygen allow us to consider the ore component to be of magmatic origin.

Platinum-group element abundances and Os isotope composition of mantle peridotites from the Mamonia complex, Cyprus

Platinum-group element (Os, Ir, Ru, Pt and Pd) abundances and Re–Os isotopic composition of fifteen peridotites (eleven spinel lherzolites and four spinel harzburgites) from the Mamonia complex, Cyprus, were determined as well as major and trace element compositions of bulk-rocks and minerals. Spinel lherzolites show excellent correlation between parameters indicating the degree of melting — e.g. Fo content in olivine, Cr# (Cr/(Cr + Al)) of Cr-spinel, Al and Yb concentrations in clinopyroxene. The degree of partial melting, calculated using Cr-spinel compositions, range from 1% to 9%. The PGE contents in spinel lherzolites show correlation with each other and with the spinel compositions, and thus can be related to the partial melting of the mantle source. The PGE abundances of the mantle source are estimated using the least depleted spinel lherzolite samples: Os 3.6 ± 0.5, Ir 3.4 ± 0.5, Ru 6.5 ± 0.9, Pt 6.1 ± 0.2, Pd 3.9 ± 0.2 (in ppb). Spinel harzburgites, despite a good correlation between the whole rock major element abundances and mineral compositions (e.g., Yb in clinopyroxene and Cr# of spinel), indicate no relationship between the Fo content of olivine and the Cr# of spinel. Hence, the harzburgites cannot be the residuum of simple partial melting, but have a more complex origin (e.g. melt percolation). Pt/Ir ratios increase in the harzburgites as Pt increases. Similar behavior of Pt and Pd is observed in abyssal and SCLM harzburgites, and explained by sulfide precipitation during melt percolation. Rhenium concentrations in most Mamonia peridotites are significantly higher than in the primitive mantle and does not show correlations with PGE. Indeed Re concentrations tend to increase with the Cr# of spinel. Thus its distribution is not governed by partial melting and we suggest that Re addition to the peridotites of Mamonia occurred during serpentinization. The Re–Os model ages of peridotites form three age clusters at 250 Ma, 600–800 Ma and > 1000 Ma. The youngest age is consistent with the age of magmatic rocks in the Mamonia Complex, whereas the “oldest” peridotites may belong to remnants of the subcontinental mantle.

Geochemistry and age of magmatic rocks in the unexposed Narromine, Cowal and Fairholme Igneous Complexes in the Ordovician Macquarie Arc, New South Wales

Much of the northern and southern sections of the Junee – Narromine Volcanic Belt of the Ordovician Macquarie Arc in central-western New South Wales are buried beneath the sediment cover of the Great Artesian Basin. Exploration drilling of these aeromagnetically defined blocks has provided important new material to assess the temporal and magmatic affinities of the rocks in the Narromine Igneous Complex (northern end) and the Cowal and Fairholme Igneous Complexes (southern end). Basement rocks are representative of Phase 1 magmatism in the Macquarie Arc, and consist of Lower Ordovician basalts and andesites and common volcaniclastic rocks, all with high-K calc-alkaline affinities. These are very similar compositionally to the Lower Ordovician Nelungaloo Volcanics that outcrop in the central part of the Junee – Narromine Volcanic Belt west of Parkes. Intruding the basement volcanic – volcaniclastic package are three distinct igneous suites. The oldest of these, dated at ca 466 – 460 Ma (Middle Ordovician) and representative of Phase 2 magmatism in the Macquarie Arc, consists of stocks of monzogabbro, monzodiorite and monzonite with high-K calc-alkaline affinities, and is well represented in the Narromine and Cowal Igneous Complexes. Phase 2 suite rocks were, in turn, intruded in the Bolindian (445 ± 5 Ma) by kilometre-size stocks, narrow sheets and dykes made up of hornblende gabbro, diorite and granodiorite, and including quartz + plagioclase + hornblende-phyric dacitic porphyries in the Narromine Igneous Complex. These medium-K calc-alkaline rocks, assigned to the Phase 3 Copper Hill Suite that also occurs in the Molong and Rockley – Gulgong Volcanic Belts further east, crystallised from andesitic or more evolved magmas, probably derived via partial melting of low-K rocks in the arc basement, or amphibolites in the subducting slab. The youngest rocks in these igneous complexes are believed to be the high-level shoshonitic intrusives in the Cowal and Fairholme Igneous Complexes, which are correlated on petrographic and compositional grounds with the Macquarie Arc Phase 4 (Bolindian and Llandovery) magmatic products best represented by the Northparkes Igneous Complex in the central part of the Junee – Narromine Volcanic Belt.

Late Miocene obduction and microplate migration around the southern Banda Sea and the closure of the Indonesian Seaway

Miocene shallowing and closure of the Indonesian Seaway between the Indian Ocean and the Pacific is related to plate-tectonic developments at the southern margins of the Banda Sea. Ultramafites, mainly lherzolite, closely associated with quartzo-pelitic metamorphic rocks, on the northern coast of Timor and on smaller islands in the southern Outer Banda Arc and also on southwest Seram in the northern Banda Arc, are fragments of Middle Miocene oceanic lithosphere obducted in the Late Miocene on sole rock of Australian continental origin. Initially cool sole rock was dynamo-thermally metamorphosed by heating from above by very hot, overriding oceanic lithosphere. Temperature equilibration between the sole and the Kaibobo lherzolitic complex (southwest Seram) took place at about 740°C and 4–5 kbar. Anatectic granitic magma formed in the sole and intruded in the ultramafite. 40Ar 39Ar determinations on muscovite and biotite from the sole and anatectic granite indicate that the system cooled through 400°C 6.0 Ma ago and through 320°C 0.5 Ma later. P-T-t modelling of the obduction/post-obduction exhumation curve, which is based on thermobarometry of the Kaibobo ultramafic complex and its sole, suggests that obduction started about 9.5 Ma, emplacement was completed about 8 Ma ago and that fast vertical movements continued until about 7 Ma. The obduction of the Kaibobo lherzolitic complex actually took place in an area northeast of the current position of Tanimbar, where Seram (as a micro-continental Australian fragment) lay 8 Ma ago, as it migrated northward with the Australian Plate. The reconstructed obduction site of the southwest Seram ultramafites and those between northern Timor and Tanimbar determine an 850 km-long, ENE-trending zone along the southern margin of the Banda Sea. As the obduction of the lherzolitic complex on northern Timor also took place 8 Ma ago and cooling to 300°C occurred 5.5 Ma ago, a similar time setting to Kaibobo is inferred. It is argued that oceanic lithosphere was being formed in the Early Miocene (about 6 Ma prior to the start of obduction) along this zone in the southern Banda Sea. The lherzolitic composition and petrographical and geochemical details suggest that the obducted ultramafites are remnants of weakly depleted, lherzolitic lithosphere, formed close to a passive margin by processes of very slow spreading. This notion and published palaeomagnetic data relating to the Neogene positions of Timor and the Australian continent as well as the types and ages of magmatic rocks in the Banda volcanic arc lead to a model for the Middle and Late Miocene geotectonic developments in the southern Banda Sea between the Eurasian and the Australian plates. The proposed model features interarc spreading in a short-lived, interarc Timor Plate (16–9.5 Ma) just north of the line Timor-Tanimbar, lengthening of the east Sunda volcanic arc by the creation of the volcanic Banda Arc, obduction-emplacement and exhumatian of ultramafic complexes, and migration of the Seram microcontinent. The model is in good agreement, in space and in time, with the 9.9–7.5 Ma history of shallowing and eventual closure of the Indonesian Seaway, as inferred in the literature from developments in biogeographic patterns and vertical thermal structure evolution of Miocene equatorial Pacific surface waters.

P21 K-Ar ages of Magmatic rocks and Geochemical Variations of Volcanics from South Andes (37°to 37°15″S-71°W)

P21 K-Ar ages of Magmatic rocks and Geochemical Variations of Volcanics from South Andes (37°to 37°15″S-71°W)

Geochemical-isotopic characteristics and K-Ar ages of magmatic rocks from Hundar valley, Shyok Suture Zone, Ladakh

DOI = 10.3126/hjs.v5i7.1229 Himalayan Journal of Sciences Vol.5(7) (Special Issue) 2008 p.18