Geochemistry of minerals of Yllymakh massif - Mesozoic alkaline ring intrusions of Central Aldan, Yakutia

<p>The Yllymakh massif is one of the Mesozoic ring intrusions of Central Aldan, Yakutia. Geological relations between rocks in this massif are enough complicated to call it multiphase. Therefore, the idea about one or different magma sources is still the topic of modern discussions. According to the previous works, there are a lot of different rocks in the Yllymakh massif. And our petrological investigation [Vasyukova et al, 2020] accepted three groups of rock that differ a lot from each other. They have not great differences in mineral composition (aegirine in all rocks, feldspars in syenites). But some critical points in their geochemical features and ages. Foid syenites containing nepheline and pseudoleucite belong to the first group. They are 140±1.9Ma old. Second group includes alkali syenites (131±2.4Ma old). And the third group of rocks are alkaline granites mostly consist of alkali pyroxene and quartz (125±1.9Ma old).</p><p>All studied rocks are divided into three groups according to the silica content and contents of the most of other elements. Points marking the composition of syenites from different groups form multidirectional trends. The alkali granite’s characteristics make an independent cluster. The REE-plots also vary. Rocks of the first group has U-shape plot and wide variations in absolute contents. Rocks of the second group have high contents of REE and gentle slope. The granites from the third group have also U-shape plot but the lowest contents.</p><p>In this work we use the LA-ICP MS to determine the contents of RE elements in minerals. There were two minerals, that have chosen – apatite and pyroxene. Usually, apatite is the main concentrator of noncoherent elements that control the form of REE-spectra and the level of REE-contents in rocks. But in the Yllymakh massif, all apatite have a similar spectra form of normalized contents. The plots of normalized REE contents have a sharp negative slope and are characterized by very insignificant Eu anomalies. Such graphs are typical for the apatite of alkaline complexes. At the same time, the REE-plots of pyroxenes are quite equal to the form of REE-plots of the corresponding rock. Pyroxenes from foid syenites and alkali granites have U-shape plot and pyroxenes from feldspar syenites have a regular negative gentle slope plot. The only difference is that the REE content in the granite pyroxenes is as high as in the syenites.</p><p>The results of the research suggest that the formation of the rock spectrum of the Yllymakh massif occurred by reactivation of geochemically similar sources in a different time in addition to others. The contents of REE in rocks were controlled by REE-contents in pyroxene and its ratio with other rock-forming minerals. Supported by RFBR grant 19-05-00788</p><p><img src="https://contentmanager.copernicus.org/fileStorageProxy.php?f=gnp.a8f2a37e3c0065515141161/sdaolpUECMynit/12UGE&app=m&a=0&c=a9f448fcade7302be1a6ca5e05985168&ct=x&pn=gnp.elif&d=1" alt=""></p><p><img src="https://contentmanager.copernicus.org/fileStorageProxy.php?f=gnp.ad91c58e3c0063715141161/sdaolpUECMynit/12UGE&app=m&a=0&c=61a6b8cf3e9847d9a2091f620d19fdc7&ct=x&pn=gnp.elif&d=1" alt=""></p><p><img src="https://contentmanager.copernicus.org/fileStorageProxy.php?f=gnp.dbd2469e3c0060915141161/sdaolpUECMynit/12UGE&app=m&a=0&c=7fc98b6a3ce7560d19ce7158c38a4079&ct=x&pn=gnp.elif&d=1" alt=""></p>

Tracking Prototethyan assembly felsic magmatic suites in southern Yunnan (SW China): evidence for an Early Ordovician–Early Silurian arc–back-arc system

Early Paleozoic trondhjemites, gneissic granites and alkali granites in southern Yunnan preserve important records of the tectonic evolution of the Prototethyan Ocean and regional correlations. Zircon ages suggest that these granitoids were emplaced from 476 to 436 Ma. The trondhjemites are characterized by high Na2O and low K2O contents, with εNd(t) values of −1.9 to −3.5 and εHf(t) values of −2.8 to +3.9. The trondhjemites were derived from an amphibolite source with a juvenile mafic component. The gneissic granites belong to the metaluminous low-K calc-alkaline series with an εNd(t) value of −6.2 and εHf(t) values of −5.0 to −0.4. The alkali granites belong to the high-K calc-alkaline series and yield εNd(t) values of −10.1 to −10.7 and εHf(t) values of −7.9 to −2.3. The gneissic granites were derived from an ‘ancient' lower mafic crust, whereas the alkali granites were derived from a meta-sedimentary source. These granitoids were formed during the subduction of the Prototethyan Ocean beneath the Simao Block and can be compared with similar igneous rocks from the Truong Son and Tam Ky-Phuoc Son zones in southern Laos. Our study, along with Early Paleozoic igneous suites from southern Laos, central Vietnam and the Malay Peninsula, suggests an arc–back-arc system along the northern margin of Gondwana.Supplementary material: Tables of zircon U–Pb and in-situ Hf and geochemical data are available at https://doi.org/10.6084/m9.figshare.c.5322386

New View on the Genesis of the Bashuihe Pluton, Laoshan Granites, China: Indications from Fluid Inclusions and H–O Isotopes

Oval caves have recently been discovered in the Bashuihe granite pluton of Laoshan Mountain, China. Oval caves typically occur in alkaline granites. This study conducted microthermometry and stable isotope analysis of quartz inclusions from oval caves and host rocks from the Bashuihe pluton to reconstruct the diagenetic evolutionary history of the Laoshan area. The temperature measurement results indicated a homogenisation temperature range from 162.5 to 261.6°C (mean 203.9°C), a salinity range of 2.1–8.3 wt% (mean 5.07 wt%), and a density range of 0.8–0.98 g/cm3 (mean 0.90 g/cm3), indicating a low-temperature, low-salinity, and low-density fluid. The emplacement depth ranged from 2.73 km to 4.43 km, indicating medium-shallow granite. A hydrogen and oxygen isotope analysis ( δ D = − 83.58 – − 67.17 , δ 18 O H 2 O = 0.83 – 0.39 ) revealed that the diagenetic fluids of the Bashuihe pluton represented a mixed hydrothermal solution composed of meteoric water and magmatic water. The results of a whole rock, H–O isotopes, rare earth element, and high field strength element analysis on the Laoshan alkali granites suggest significant hydrothermal activity in the late stage of magmatism. Primary oval caves in the Bashuihe pluton most likely evolved in the following sequence: fluid was enriched in the late diagenetic stage, diagenetic minerals crystallised under low temperature and pressure conditions, the crystallisation rate accelerated, and the magma condensed rapidly. Moreover, the increase in magma fluid enabled the movement and convergence of fluid. The accumulated fluid and volatiles occupied more space, and rapid magma condensation trapped the accumulated fluid and volatiles in the pluton, forming the oval granite cave. This research provides a crucial theoretical reference for the development and utilisation of underground space and engineering buildings in granite regions.

Lithostratigraphy of the Mesoproterozoic Yas-Schuitdrift Batholith, South Africa and Namibia

The granitic and leucogranitic Yas and Schuitdrift Gneisses occur together as a large ovoid pre-tectonic batholith that crosses the Orange River border between South Africa and Namibia. They occur in the central parts of the Kakamas Domain in the Namaqua Sector of the Namaqua-Natal Metamorphic Province where they intrude, and are deformed together with, slightly older (~1.21 Ga) orthogneisses and granulite-facies metapelitic gneisses. The Yas Gneiss occurs mainly on the outer perimeter and northern parts of the batholith and comprises equigranular leucogranite gneiss and biotite granite augen orthogneiss, whereas the Schuitdrift biotite-hornblende augen gneiss is located at the centre and southern parts of the batholith. The batholith is strongly deformed with penetrative Namaqua-aged gneissic fabrics defined by grain-flattening of quartz and feldspar in the equigranular leucogneisses and aligned K-feldspar megacrysts in the augen gneisses. The gneissic fabric is refolded during a large-scale folding event that results in the dome-shape of the batholith and controls the present outcrop pattern of its various components. Flexure along the margins of the batholith refoliated the gneisses into a zone of mylonitic rocks. The Yas and Schuitdrift Gneisses have similar geochemistry and classify as alkali granites and alkali leucogranites. They are felsic (mean SiO2: 74.5 wt%) and potassic (mean K2O: 5.8 wt%) but have low MgO, CaO and Na2O, reflecting their low mafic mineral and plagioclase contents. The Schuitdrift Gneiss yielded U-Pb zircon ages of 1 191 ± 7 and 1 187 ± 6 Ma.

Geochemistry and origin of a new-type of Zr-Nb-Y-REE deposit in highly-evolved alkali granite, Chungju area, South Korea

<p>In the Chungju area, Korea, highly enriched Zr-Nb-Y-REE deposits occur in subhorizontal layered Paleozoic granitic rocks (331±1.5 Ma), which can be divided into layered alkali granite, alkali aplite, and pegmatite. The rocks are mainly composed of alkali feldspar, quartz, and microcrystalline zircon. The ubiquitous zircon is the distinctive feature of the alkaline rocks, which plot with within plate granite, anorogenic granite, and ultrapotassic rocks, and show very similar REE patterns. Alkali aplite has especially high total rare earth elements and negative europium anomalies compared to the layered alkali granite. The Zr-Nb-Y-REE mineralization occurs as zircon-magnetite bands that are associated with several REE minerals. Repeated graded textures of layered alkali granites with interlayered Zr-Nb-Y-REE mineralization can be explained by gravity accumulation in the late magmatic stage. The compositions of zircons plot between the late magmatic and hydrothermal fields. The REE patterns of zircon-rich mineralization shows slightly negative slopes, whereas zircons show positive slopes. This can be explained by the HREE being strongly partitioned into zircon grains from the melt. Zircons with low total REE contents show high positive Ce anomalies. Although zircon analyses were conducted on one sample from a small area, it shows variable Ce anomalies and TREE, which indicates the zircons crystallized under conditions of rapidly changing oxygen fugacity, as the REE contents of zircon are related to the oxygen fugacity of the melt. The limited Th/U ratios of zircons indicate that they crystallized during a simple magmatic event, and were not affected by hydrothermal alteration and metamorphism. Here we suggest a flotation, aggregation, and gravity accumulation model can explain settle down of microcrystalline zircon and magnetite grains in fluid rich alkaline melt. This is the first report on highly evolved alkali granite that associated with Zr-Nb-Y-REE mineralization. The features displayed in these deposits have important implications for the evolution of alkali magmas.</p>

Tectono-magmatic evolution of the younger Gardar southern rift, South Greenland

The 1300–1140 Ma Gardar period in South Greenland involved continental rifting, sedimentation and alkaline magmatism. The latest magmatism was located along two parallel rift zones, Isortoq–Nunarsuit in the north and the Tuttutooq–Ilimmaasaq–Narsarsuaq zone in the south addressed here. The intrusive rocks crystallised at a depth of troctolitic gabbros. These relatively reduced magmas evolved through marked iron enrichment to alkaline salic differentiates. In the Older giant dyke complex, undersaturated augite syenites grade into sodalite foyaite. The larger, c . 1163 Ma Younger giant dyke complex (YGDC) mainly consists of structureless troctolite with localised developments of layered cumulates. A layered pluton (Klokken) is considered to be coeval and presumably comagmatic with the YGDC. At the unconformity between the Ketilidian basement and Gardar rift deposits, the YGDC expanded into a gabbroic lopolith. Its magma may represent a sample from a great, underplated mafic magma reservoir, parental to all the salic alkaline rocks in the southern rift. The bulk of these are silica undersaturated; oversaturated differentiates are probably products of combined fractional crystallisation and crustal assimilation. A major dyke swarm 1–15 km broad was intruded during declining crustal extension, with decreasing dyke widths and increasing differentiation over time. Intersection of the dyke swarm and E–W-trending sinistral faults controlled the emplacement of at least three central complexes (Narssaq, South Qôroq and early Igdlerfigssalik). Three post-extensional complexes (Tugtutôq, Ilímaussaq and late Igdlerfigssalik) along the former rift mark the end of magmatism at c . 1140 Ma. The latter two complexes have oblate plans reflecting ductile, fault-related strain. The Tugtutôq complex comprises quartz syenites and alkali granites. The Ilímaussaq complex mainly consists of nepheline syenite crystallised from highly reduced, Fe-rich phonolitic peralkaline (agpaitic) magma, and resulted in rocks with very high incompatible element concentrations. Abundant anorthositic xenoliths in the mafic and intermediate intrusions point to a large anorthosite protolith at depth which is considered of critical importance in the petrogenesis of the salic rocks. Small intrusions of aillikite and carbonatite may represent remobilised mantle metasomites. The petrological similarity between Older and Younger Gardar suites implies strong lithospheric control of their petrogenesis. The parental magmas are inferred to have been derived from restitic Ketilidian lithospheric mantle, metasomatised by melts from subducting Ketilidian oceanic crust and by small-scale melt fractions associated with Gardar rifting. There are numerous analogies between the southern Gardar rift and the Palaeogene East African rift.