Journal of Earth Science

CONTENTS

2021, 32(6): .

[Abstract](81) [PDF 194KB](59)

Abstract:

Preface

Maria de Fátima Bitencourt, Valdecir de Assis Janasi

2021, 32(6): 1287-1288. doi: 10.1007/s12583-021-1320-7

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Dehydration Melting and Proterozoic Granite Petrogenesis in a Collisional Orogen—A Case from the Svecofennian of Southern Finland

Tom Andersen, O. Tapani Rämö

2021, 32(6): 1289-1299. doi: 10.1007/s12583-020-1385-8

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Dehydration melting of metasupracrustal rocks at mid-to deep-crustal levels can generate water undersaturated granitic melt. In this study, we evaluate the potential of ~1.89-1.88 Ga metasupracrustal rocks of the Precambrian of southern Finland as source rocks for the 1.86-1.79 Ga late-orogenic leucogranites in the region, using the Rhyolite-MELTS approach. Melt close in composition to leucogranite is produced over a range of realistic pressures (5 to 8 kbar) and temperatures (800 to 850℃), at 20%-30% of partial melting, allowing separation of melt from unmelted residue. The solid residue is a dry, enderbitic to charnoenderbitic ganulite depleted in incompatible components, and will only yield further melt above 1 000-1 050℃, when rapidly increasing fractions of increasingly calcic (granodioritic to tonalitic) melts are formed. The solid residue after melt extraction is incapable of producing syenogranitic magmas similar to the Mid-Proterozoic, A-type rapakivi granites on further heating. The granitic fraction of the syenogranitic rapakivi complexes must thus have been formed by a different chain of processes, involving mantle-derived mafic melts and melts from crustal rock types not conditioned by the preceding late-orogenic Svecofennian anatexis.

A Comparison between the ~1.08-1.13 Ga Volcano-Sedimentary Koras Group and Plutonic Keimoes Suite: Insights into the Post-Collisional Tectono-Magmatic Evolution of the Eastern Namaqua Metamorphic Province, South Africa

Russell Bailie, Aidan Leetz

2021, 32(6): 1300-1331. doi: 10.1007/s12583-021-1462-7

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Along the eastern margin of the Mesoproterozoic Namaqua metamorphic province (NMP) of southern Africa are a bimodal volcano-sedimentary succession, the ~1.13-1.10 Ga Koras Group, composed of rhyolitic porphyries and basaltic andesites, and the ~1.11-1.07 Ga late-to post-tectonic granitoids of the Keimoes Suite. This review examines existing whole-rock major-and trace-element data, along with isotope chemistry (with some new isotopic data), to investigate the role these two magmatic successions played in terms of post-collisional magmatism of the eastern NMP near the boundary with the Archean Kaapvaal Craton. The Keimoes Suite comprises variably porphyritic biotite monzogranites and granodiorites, with a charnockitic member. They are metaluminous to weakly peraluminous, ferroan, and calc-alkalic. They exhibit large ion lithophile (LIL) element enrichment relative to the high field strength elements (HFSE) with depletions in Ba, Sr, Nb, P, Eu and Ti, and enrichments in Th, U and Pb. Isotopic values (e_Nd(t): 2.78 to -2.95, but down to -8.58 for one granite, depleted mantle Nd model ages (T_DM): 1.62-1.99 Ga, but up to 2.55 Ga; initial ⁸⁷Sr/⁸⁶Sr: 0.652 82-0.771 30) suggest derivation from weakly to mildly enriched (and radiogenic) sources of Meso-to Paleoproterozoic age, the former of more juvenile character. The Koras Group is characterized by a bimodal succession of calcic to calc-alkalic, magnesian and tholeiitic basaltic andesites and mostly metaluminous to peralkaline rhyolitic porphyries. Two successions are recognised, an older, lower succession that extruded at ~1.13 Ga, and a younger, upper succession at ~1.10 Ga. The rhyolitic porphyries of both successions show similar LILE/HFSE enrichment and the same element enrichments and depletions as the Keimoes Suite granitoids. The upper succession is consistently more fractionated in terms of both whole-rock major and trace element chemistry, and, isotopically, has a greater enriched source component (e_Nd(t): -0.69 to -4.26; T_DM: 1.64-2.44 Ga), relative to the lower succession (e_Nd(t): 0.74-5.62;T_DM: 1.28-2.12 Ga). Crystal fractionation of plagioclase and K-feldspar appears to have played a role in bringing about compositional variation in many of the granites. These were derived from partial melting of mainly igneous with subordinate sedimentary sources from mostly lower crustal depths, although some granitoids have indications of a possible mantle source component. The lower succession of the Koras Group was derived by partial melting of subduction-influenced enriched mantle giving rise to mafic magmas that fractionated to give rise to the rhyolitic porphyries. The upper succession rhyolites were derived by crustal melting due to the input of mafic magmatism. Crystal fractionation was the main compositional driver for both successions. The Keimoes Suite granitoids and the Koras Group are associated with extensional regimes subsequent to the main deformational episode in the eastern NMP.

Carbonatites and Alkaline Igneous Rocks in Post-Collisional Settings: Storehouses of Rare Earth Elements

Kathryn M. Goodenough, Eimear A. Deady, Charles D. Beard, Sam Broom-Fendley, Holly A. L. Elliott, Frederick van den Berg, Hüseyin Öztürk

2021, 32(6): 1332-1358. doi: 10.1007/s12583-021-1500-5

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The rare earth elements (REE) are critical raw materials for much of modern technology, particularly renewable energy infrastructure and electric vehicles that are vital for the energy transition. Many of the world's largest REE deposits occur in alkaline rocks and carbonatites, which are found in intracontinental, rift-related settings, and also in syn-to post-collisional settings. Post-collisional settings host significant REE deposits, such as those of the Mianning-Dechang belt in China. This paper reviews REE mineralization in syn-to post-collisional alkaline-carbonatite complexes worldwide, in order to demonstrate some of the key physical and chemical features of these deposits. We use three examples, in Scotland, Namibia, and Turkey, to illustrate the structure of these systems. We review published geochemical data and use these to build up a broad model for the REE mineral system in post-collisional alkaline-carbonatite complexes. It is evident that immiscibility of carbonate-rich magmas and fluids plays an important part in generating mineralization in these settings, with REE, Ba and F partitioning into the carbonate-rich phase. The most significant REE mineralization in post-collisional alkaline-carbonatite complexes occurs in shallow-level, carbothermal or carbonatite intrusions, but deeper carbonatite bodies and associated alteration zones may also have REE enrichment.

Shoshonitic Magmatic Series and the High Ba-Sr Granitoids: A Review with Emphasis on Examples from the Neoproterozoic Dom Feliciano Belt of Southern Brazil and Uruguay

Lauro Valentim Stoll Nardi, Maria de Fátima Bitencourt, Luana Moreira Florisbal, Dionatan Ferri Padilha

2021, 32(6): 1359-1373. doi: 10.1007/s12583-021-1534-8

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Fractional crystallization of parental magmas of shoshonitic or silica-saturated, ultrapotassic affinity, with variable amount of concurrent crustal assimilation, may result in granitic and syenitic rocks. Typical plutonic members of the shoshonitic series are monzonites and quartz monzonites, whilst syenites and quartz syenites are the dominant plutonic products of the ultrapotassic series. Lamprophyric magmas are commonly found in association with both series and are frequently part of coeval mingling/mixing systems. Ultrapotassic and shoshonitic primary magmas, including lamprophyric ones, are derived from amphibole-phlogopite-bearing mantle sources produced by previous, subduction-related metasomatism. Acidic and intermediate rocks can be derived from such parental magmas, generally through AFC processes. Shoshonitic-like granitoids, which have not clear relation with intermediate or basic shoshonitic rocks, or are produced dominantly by crustal melting, should be named high-Ba-Sr granitoids. This study focuses mainly on Neoproterozoic shoshonitic and silica-saturated ultrapotassic rock associations formed in post-collisional settings from southern Brazil and Uruguay. The source of magmas, their evolution, the role played by crustal contamination in modifying pristine geochemical signatures and their tectonic control are discussed based on elemental and Sr-Nd isotope geochemistry. The main features of plutonic rocks related to the shoshonitic series are their potassic, silica-saturated alkaline character, predominance of monzonitic to syenitic compositions, high Sr and Ba contents, monotonous, light REE-enriched patterns, and moderate HFSE contents. Syntectonic shoshonitic and high Ba-Sr granitoids within shear zones show lower alkali, LREE, HFSE, and Sr contents than those formed away from the main deformation sites. Plutonic rocks related to the extended silica-saturated ultrapotassic series are mostly syenites, alkali-feldspar granites and lamprophyres with K₂O/Na₂O ratios above 2. The typical values of ⁸⁷Sr/⁸⁶Sr_i for shoshonitic plutonic rocks are 0.706-0.708, ranging from 0.704 to 0.710. The ε_Nd(t) values are negative and vary from 0 to -24. Crustal contribution tends to increase ⁸⁷Sr/⁸⁶Sr_i and decrease ε_Nd(t) values, depending on protolith isotope signature, melting conditions and volume of assimilated material. Ultrapotassic rocks, on the other hand, show higher ⁸⁷Sr/⁸⁶Sr_i ratios, from 0.709-0.711 up to 0.720. Geochemical evidence, including Sr-Nd isotope data, indicates that the shoshonitic and ultrapotassic rocks discussed in this study were formed from OIB-like sources with strong influence of previous subduction, probably a phlogopite, K-amphibole bearing veined mantle. Lithological variability in ultrapotassic-shoshonitic associations is interpreted to result from (ⅰ) variation of source composition, (ⅱ) different melt fractions from similar sources, (ⅲ) mixing-mingling, fractional crystallization, and assimilation processes.

Genesis of the Gentio Metagranitoid: Post-Collisional High-K Plutonism within the Mineiro Belt, São Francisco Craton, Brazil

Marize M. da Silva, Ciro A. Ávila, Felipe M. Tavares, Natali S. Barbosa, Wilson Teixeira

2021, 32(6): 1374-1396. doi: 10.1007/s12583-021-1469-0

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The Gentio metagranitoid presents equigranular and porphyritic facies, modal composition ranging from tonalite to monzogranite with calculated T_Zr < 800℃ for most samples. Its mineralogy is dominated by quartz and feldspar (77% to 95%), biotite is the only mafic mineral present (2% to 18%) and, titanite, zircon, apatite, allanite are important accessory phases. These rocks range from metaluminous to weakly peraluminous, and have large variation in major and trace elements, and high alkali contents (> 6 wt.%). Zircon analyses by LA-ICP-MS and SHRIMP yielded a concordia age of 2 119±10 Ma for the porphyritic facies and an upper intercept age of 2 111±15 Ma for the equigranular facies. The whole-rock Sm-Nd T_DM ages vary from 2.4 to 2.8 Ga with ε_Nd(2.1) values between -0.7 and -5.3, indicating crustal derivation from distinct and/or heterogeneous protoliths. Field observations indicate that the Gentio metagranitoid was formed through different pulses of magma. Individual batches were subject to little or even no fractionation process after its emplacement. Although the Gentio metagranitoid crosscuts metamafic and metaultramafic rocks akin to an oceanic arc setting, this pluton is likely originated by partial melting of a more evolved quartz-feldspathic crustal igneous rock in a post-collisional environment, after the accretion of the arcs from the Mineiro belt and rocks of the Mantiqueira Complex.

Petrogenesis of Granites from the Ediacaran Socorro Batholith, SE Brazil: Constraints from Zircon Dating, Geochemistry and Sr-Nd-Hf Isotopes

Bárbara Bueno Toledo, Valdecir de Assis Janasi

2021, 32(6): 1397-1414. doi: 10.1007/s12583-021-1494-z

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Whole rock elemental and Sr-Nd isotope geochemistry and in situ zircon Hf isotope geochemistry were used to identify the sources of the Neoproterozoic granites from the Socorro batholith, Socorro-Guaxupé Nappe (SGN), South Brasilia Orogen, Brazil. Zircon trace elements and Hf isotope geochemistry provided information about sources and crystallization (T, f_O₂) conditions. Three main types of granites built the bulk of the batholiths, beginning with probably pre-collisional ~640-630 Ma charnockites, and ending with ~610 Ma voluminous post-collisional high-K calc-alkaline (HKCA) I-type granites (Bragança Paulista-type). Several types of leucogranites were generated from 625 to 610 Ma, spanning the interval from collisional to post-collisional tectonics. Two charnockite bodies occur in the study area: the ~640 Ma Socorro charnockite has remarkable chemical similarities with Bragança Paulista-type granites, but higher ε_Nd(t)=-6.1 and average zircon ε_Hf(t)=-9.1 and lower ⁸⁶Sr/⁸⁷Sr_t (0.709 3) values, indicative of more juvenile and water-poor source. The ~633 Ma Atibaia charnockite has distinct geochemical signature (lower Mg^# and Sr content; higher Zr), more negative ε_Nd(t)=-14.1, similar average zircon ε_Hf(t)=-8.9, and much higher ⁸⁶Sr/⁸⁷Sr_t=0.719 7, probably reflecting a larger component from old crust. The predominant ~610 Ma Bragança Paulista-type granites were emplaced in a post-collisional setting, and correspond to porphyritic biotite-hornblende monzogranites of high-K calc-alkaline character with 61 wt.%-67 wt.% SiO₂, high Mg^# (39-42), Sr/Y (19-40), La/Yb (12-69), highly negative ε_Nd(t) (-12.3 to-12.9) and zircon ε_Hf(t) (-12 to -17) and ⁸⁷Sr/⁸⁶Sr_t=0.711 9-0.713 1. These features are interpreted as indicative of magma generation in a thickened crust, where melts from enriched mantle sources emplaced in the lowermost crust, heated host old continental crust rocks (gneisses and granulites) and partially mixed with their melting products. Leucogranite plutons (SiO₂ > 72 wt.%) occurring in the southern portion of the batholith have a range of geochemical and isotope signatures, reflecting melting of crustal sources in space and time between ~625 Ma (Bocaina Pluton) and ~610 Ma (Bairro da Pedreira Pluton). Highly negative ε_Nd(t) (-16.2) and average zircon ε_Hf(t)=-16, and high ⁸⁷Sr/⁸⁶Sr_t(0.715 6-0.717 1) are consistent with relatively old ortho-and paragneiss sources similar to those which generated regionally abundant migmatites and anatectic granites in the collisional to post-collisional setting.

Origin of the Low δ¹⁸O Signals in Zircons from the Early Cretaceous A-Type Granites in Eastern China: Evidence from the Kulongshan Pluton

Wenbo Fan, Neng Jiang, Mingguo Zhai, Jun Hu

2021, 32(6): 1415-1427. doi: 10.1007/s12583-021-1515-y

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The origin of low δ¹⁸O signals in zircons from the Early Cretaceous A-type granites in eastern China has long been disputed. It is uncertain whether the ¹⁸O-depleted features were inherited from high-temperature hydrothermal altered source rock or resulted from water-rock interaction after emplacement. In this paper, zircon oxygen isotopes in the ~130 Ma Kulongshan A-type granites in the northern North China Craton are analyzed. The zircons could be subdivided into 5 types based on their luminescent intensity and internal structures in CL images. Their δ¹⁸O values also vary in different types and show negative correlation with U and Th contents and accompanying cumulative α-decay doses, implying that their δ¹⁸O values may have been modified to various degrees by meteoric water-rock interaction after the accumulation of radiation damage. The idea is further confirmed by oxygen isotopic equilibrium calculation between co-existing mineral pairs. It is inferred that only the least-influenced zircons, with slightly elevated δ¹⁸O values than normal mantle, have preserved the magmatic oxygen isotopes. In combination with other evidences, it is proposed that the A-type granites are lower-crustal-derived, unnecessarily invoking a high-temperature hydrothermal altered source. The proposition is applicable to many other Cretaceous A-type granites that have similar zircon behaviors.

Geochemical and Nd-Sr Isotopic Compositions of Hypabyssal Adakites in the Torud-Ahmad Abad Magmatic Belt, Northern Central Iran Zone: Analysis of Petrogenesis and Geodynamic Implications

Fazilat Yousefi, Ryan D. Mills, Mahmoud Sadeghian, David R. Lentz, Christina Wanhainen, Habibollah Ghasemi, Laicheng Miao

2021, 32(6): 1428-1444. doi: 10.1007/s12583-020-1378-7

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Eocene intermediate to felsic subvolcanic rocks of the Torud-Ahmad Abad magmatic belt (TAMB), in the northern part of the Central Iran zone, are exposed between the Torud and Ahmad Abad regions in South-Southeast Shahrood. These igneous rocks include hypabyssal dacite, trachyte, andesite, trachy-andesite, and basaltic andesite; they are mainly composed of phenocrysts and microcrystalline groundmass of pyroxene, amphibole, and plagioclase, with minor biotite and titanomagnetite; they form domal structures (plugs and stocks), dikes, and sills that intruded into Neoproterozoic to cogenetic Eocene volcano-sedimentary sequences. Based on isotopic analysis of these intermediate to acidic rocks, initial ratios of ¹⁴³Nd/¹⁴⁴Nd range from 0.512 775 to 0.512 893 and initial ratios of ⁸⁷Sr/⁸⁶Sr range from 0.703 746 to 0.705 314, with quite positive ɛ_Nd(i)values of +3.69 to +6.00. They are enriched in light rare earth elements and large ion lithophile elements and depleted in heavy rare earth elements and high-field strength elements, the SiO₂ content is (52-62) wt.%, and Na₂O content > 3 wt.%, Al₂O₃ content > 16 wt.%, Yb < 1.8 ppm, and Y < 18 ppm. These geological, geochemical, and Sr and Nd isotopic data are consistent with adakitic signatures originating by partial melting of the subducted Neo-Tethys oceanic slab (Sabzevar branch) and lithospheric suprasubduction zone mantle. The mantle signatures typifying the rapidly emplaced adakitic rocks (slab (high-silica adakite) and suprasubduction zone (low-silica adakite) melts) together with their locally voluminous extent are evidences that support a locally extensional geodynamic setting; and the evidence is consistent with an evolution to local transpression in the Late Eocene in this convergent margin arc environment to rifting (basalts to adakites) towards submarine conditions in the Neogene.

Melt-Lithosphere Interaction Controlled Compositional Variations in Mafic Dikes from Fujian Province, Southeastern China

Zhuliang Lei, Gang Zeng, Jianqiang Liu, Xiaojun Wang, Lihui Chen, Xiaoyu Zhang, Jinhua Shi

2021, 32(6): 1445-1453. doi: 10.1007/s12583-020-1358-y

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Late Mesozoic magmatism in southeastern China has been widely considered to be related to the subduction of the Paleo-Pacific Plate. However, it remains controversial whether mafic rocks are derived from the lithosphere or the asthenosphere. Here we present a comprehensive study on mafic dikes from Fujian Province in southeastern China, aiming to understand their source. Two types of mafic rocks have been recognized based on their trace-element features. Type-I rocks show arc-like trace-elemental characteristics, while type-II rocks are distinguished by their relatively flat patterns in primitive-mantle-normalized trace-element diagram. Despite such differences between two types of rocks, these mafic dikes show two trends in the plots of ⁸⁷Sr/⁸⁶Sr_(i) versus La/Nb, which can be explained by the influences of crustal contamination and melt-lithospheric mantle interaction, respectively. ⁸⁷Sr/⁸⁶Sr_(i), La/Nb, Sr/Y and Zr/Y ratios of type-I rocks are significantly correlated to the thickness of the underlying lithosphere, and the signals of lithosphere are clearer with increasing lithospheric thickness. This highlights the important influences of melt-lithosphere interaction during their formation. Such observations also indicate that these mafic rocks are more likely to have been originated from the asthenosphere rather than the lithospheric mantle.

Magmatic Evolution of Garnet-Bearing highly Fractionated Granitic Rocks from Macao, Southeast China: Implications for Granite-Related Mineralization Processes

Pedro Quelhas, João Mata, Ágata Alveirinho Dias

2021, 32(6): 1454-1471. doi: 10.1007/s12583-020-1389-4

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The widespread W-(Mo)-Sn-Nb-Ta polymetallic mineralization in Southeast (SE) China is genetically associated with Mesozoic highly fractionated granitic rocks. Such rocks have enigmatic mineralogical and geochemical features, making its petrogenesis an intensely debated topic. To better understand the underlying magma evolution processes, petrography, garnet chemistry and whole-rock major and trace element data are reported for Jurassic highly fractionated granitic rocks and associated microgranite and aplite-pegmatite dikes from Macao and compared with coeval similar granitic rocks from nearby areas in SE China. Despite the fact that the most evolved rocks in Macao are garnet-bearing aplite-pegmatite dikes, the existence of coeval two-mica and garnet-bearing biotite and muscovite granites displaying more evolved compositions (e.g., lower Zr/Hf ratios) indicates that the differentiation sequence reached higher degrees of fractionation at a regional scale. Although crystal fractionation played an important role, late-stage fluid/melt interactions, involving F-rich fluids, imparted specific geochemical characteristics to Macao and SE China highly fractionated granitic rocks such as the non-CHARAC (CHArge-and-RAdius-Controlled) behavior of trace elements, leading, for example, to non-chondritic Zr/Hf ratios, Rare Earth Elements (REE) tetrad effects and Nb-Ta enrichment and fractionation. Such process contributed to the late-stage crystallization of accessory phases only found in these highly evolved facies. Among the latter, two populations of garnet were identified in MGI (Macao Group I) highly fractionated granitic rocks: small grossular-poor euhedral grains and large grossular-rich skeletal garnet grains with quartz inclusions. The first group was mainly formed through precipitation from highly evolved Mn-rich slightly peraluminous melts under low-pressure and relatively low temperature (~700 ℃) conditions. Assimilation of upper crust metasedimentary materials may have contributed as a source of Mn and Al to the formation of garnet. The second group has a metasomatic origin related to the interaction of magmatic fluids with previously crystallized mineral phases and, possibly, with assimilated metasedimentary enclaves or surrounding metasedimentary strata. The highly fractionated granitic rocks in Macao represent the first stage in the development of granite-related W-(Mo)-Sn-Nb-Ta mineralization associated with coeval more evolved lithotypes in SE China.

New Insights into Neoproterozoic-Cretaceous Events in the Mamfe Basin (SW Cameroon, Central Africa): Evidence from Textural Analyses, U-Th Composition, and U-Pb Zircon Geochronology from Granitic Basement

Nguo S. Kanouo, Arnaud P. Kouské, David R. Lentz, Rose F. Yongué

2021, 32(6): 1472-1484. doi: 10.1007/s12583-020-1395-6

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Backscattered electron BSE-CL images, U-Th abundance, and U-Pb ages were obtained for zircons from the Nkogho granitic basement rocks cropping out in the Mamfe Basin (SW Cameroon). These data are used to characterize and classify each zircon, elucidate their geochemical traits. They were also used to formulate a model of their host composition to ascertain the source, and document any preserved post-emplacement events. These zircons mainly form long to short prisms that are pyramidal to dipyramidal in shape. They typically exhibit complex oscillatory growth zoning, as well as exhibit sector zoning. These features are mainly compatible with igneous zircons, although a few examples have metamorphic signatures. The U (30 ppm-6 380 ppm), Th (4 ppm-1 280 ppm), and Pb (12 ppm-648 ppm) contents show core to rim variations with most values fall within the range of crustal granitic zircons. The Th/U ratios (0.08-1.23), with core to rim differences mainly encompass values typical of magmatic zircons with a few values characterizing metamorphic zircons that grew in equilibrium with an anatectic melt. The U-Pb ages (108.4±1.7 to 988.4±19.0 Ma) with some core and rim age differences date Early Neoproterozoic, Cryogenian-Ediacarian, Early Cambrian-Ordovician, Devonian-Carboniferous, and Aptian-Albian events. The arc-like Nkogho I-type granitoid crystallized from granitic magma during Cryogenian to Ediacarian times and was later affected by post-Ediacarian Cambrian to Albian magmatic events. Aptian-Albian ages probably reflect opening of the Mamfe Basin.

Syn-Faulting Calcite Ages: Constraint for the Late Mesozoic Deformation of the Ningzhen Mountain, Eastern China

Jinbao Su, Hongbing Tan, Yuexing Feng, Guoxing Qin

2021, 32(6): 1485-1495. doi: 10.1007/s12583-020-1107-2

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It is difficult to date a brittle fault because of the absence of effective dating methods. Recently, calcite U-Pb dating has drawn the attention of many researchers and has been successfully applied to constrain the age of brittle deformation. The South China Block (SCB) has experienced multiphase deformations characterized by widespread brittle faults and folds, which has led to significant debate on the Mesozoic dynamic mechanism and deformational phases. A syn-faulting calcite vein that occurred in the Permian limestones of the Ningzhen Mountain region was analyzed using U-Pb dating and geochemistry techniques in this paper. The U-Pb dating results display multiphase activities with two mixed ages of 109.9±5.8 Ma (MSWD=46, n=231) and 117.2±2.4 Ma (MSWD=3.6, n=150). The age of 117.2±2.4 Ma likely represents the timing of the brittle faults. Trace elements and rare earth elements data indicate that these fault-related calcites are mainly derived from host limestones and unrelated to Cretaceous magmatic activity. These faults formed in a tectonic reactive period and dormant time of the adakitic magma in the Lower Yangtze region.

Pressure-Temperature Evolution of a Mylonitic Gneiss from the Lower Seve Nappe in the Handöl Area, Central Sweden

Botao Li, Hans-Joachim Massonne, Xiaoping Yuan

2021, 32(6): 1496-1511. doi: 10.1007/s12583-021-1413-3

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Ultrahigh-pressure metamorphism has recently been reported from various crustal rocks in the Seve Nappe Complex (SNC) in which microdiamonds were found. However, in gneiss from the Lower Seve Nappe (LSN), neither any direct petrographic indication for UHP was reported nor the metamorphic evolution was well constrained. We studied a mylonitic gneiss from the Handöl area of the LSN and applied phase-diagram modeling and Ti-in-biotite thermometry. Based on the compositions of garnet and biotite and observed mineral assemblages, a path was reconstructed passing through about 8 kbar and 730℃ at prograde metamorphism. Peak-pressure and initial retrograde stages occurred at 9.0-10.2 kbar at 745-775℃, and 7-9 kbar at < 750℃, respectively. No ultrahigh-pressure evidence was recognized compatible with medium-pressure metamorphism deduced in earlier studies of gneiss from the SNC. As higher peak pressures were reported recently for metamorphic rocks of the LSN, a possible interpretation is that slices or erased blocks were subducted, metamorphosed at different depths, and exhumed in a subduction channel. However, the dominant gneiss of the SNC experienced only a medium-pressure metamorphism in the upper part of the downgoing Baltica Plate. Rocks from different depth levels were brought together in an exhumation channel located between Baltica and the overlying plate.

Geological, Structural and Mineralogical Approach to Investigate the Evolution of Low- and very Low-Grade Metamorphic Units from the Intra-Pontide Suture Zone, Central Pontides, Turkey

Alessandro Ellero, Chiara Frassi, Mehmet Cemal Göncüoǧlu, Marco Lezzerini, Michele Marroni, Giuseppe Ottria, Luca Pandolfi, Kaan Sayit, Marco Tamponi

2021, 32(6): 1512-1527. doi: 10.1007/s12583-021-1445-8

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Abstract:
A multidisciplinary approach combining multiscale geological-structural analyses (from field to microscale) and clay mineral transformations (clay mineralogy, illite and chlorite "crystallinity" and b cell dimension (b₀) of K-white mica) was adopted to unravel the tectono-metamorphic evolution of low-and very low-grade tectono-metamorphic units from the Intra-Pontide suture zone in northern Turkey. The mineralogical study allowed to better evaluate the structural stacking outlined during the geological mapping, leading to distinguish three tectono-metamorphic units, two in epizone (Daday and Emirkӧy units) and one in anchizone (Çifter Unit) metamorphic conditions. The mesostructural and microstructural analyses suggest a polyphase tectonic-metamorphic evolution. The different macroscopic features observed between the two units characterized by the same epizone metamorphism, can be justified by the evidence that these metamorphic conditions were acquired during the last stages of exhumation in the Daday Unit, while they constrained the metamorphic-peak conditions in the Emirkӧy Unit. Contemporary analyses and comparison between structural and mineralogical data have thus proven to be a powerful tool to investigate the low-grade and very low-grade metamorphic environments, allowing at the same time to solve the apparent contradictions deriving from the mineralogical study and to significantly improve the detail of the geological mapping in the field.

Geochemical Features of Bellara Trap Volcanic Rocks of Chitradurga Greenstone Belt, Western Dharwar Craton, India: Insights into MORB-BABB Association from a Neoarchean Back-Arc Basin

Chakravadhanula Manikyamba, Sohini Ganguly, Arijit Pahari

2021, 32(6): 1528-1544. doi: 10.1007/s12583-021-1472-5

[Abstract](276) [FullText HTML] (240) [PDF 7862KB](58)

Abstract:
This study presents a comprehensive account of the petrogenetic and geodynamic evolution of the Bellara Trap volcanic rocks from the Ingaldhal Formation, Chitradurga Group, western Dharwar Craton (WDC). Geochemical attributes of these rocks are consistent with two groups with distinct evolutionary trends: one comprising tholeiitic, MORB (mid-ocean ridge basalt) type basalts (BTB) and the other corresponding to calc-alkaline andesites (BTA). Basalts are essentially composed of clinopyroxene and plagioclase whereas the andesites are porphyritic with phenocrysts of plagioclase, clinopyroxene and polycrystalline quartz embedded in a groundmass of K-feldspar, quartz and opaques. Primary igneous mineralogy is overprinted by greenschist facies metamorphism resulting in chlorite-actinolite-plagioclase assemblage. The BTB samples reflect nearly flat REE patterns with weak LREE enrichment in contrast to pronounced LREE enhancement over HREE discernible for BTA. Tectonically, the BTB samples correspond to an active mid-oceanic ridge-rift setting with a MORB composition, whereas a back-arc basin (BAB) regime is corroborated for the BTA samples fractionating from back-arc basin basalts. Geochemical imprints of subduction input are more pronounced in BTA compared to BTB as mirrored by their elevated abundances of incompatible fluid mobile elements like Ba, Th, U and LREE. The BTB is endowed with an N-to E-MORB signature attributable to minor contributions from subduction-related components at the inception of a back-arc basin in the vicinity of an active subduction system. The BTA derived through differentiation of a basaltic magma with BABB (back-arc basin basalt) affinity compositionally akin to a heterogeneous source mantle carrying depleted MORB-type and enriched arc-type components inducted with progressive subduction. The BABB-type andesites and MORB-type basalts from Bellara Traps record a compositional heterogeneity of mantle in an intraoceanic arc-back arc system. Mantle processes invoke a BABB-MORB spectrum with a MORB-like endmember and an arc-like endmember associated with a juvenile back-arc basin. This study infers a Neoarchean analogue of Mariana-type back-arc rift setting proximal to the arc with a gradual transition from anhydrous to hydrous melting processes synchronized with MORB-mantle and arc-mantle interaction during initiation of a nascent back arc adjacent to the arc. The MORB-BABB compositional spectrum for the Bellara Traps conforms to a Neoarchean back-arc basin that evolved under an extensional tectonic regime associated with incipient stages of back-arc rifting and incorporation of subduction-derived components in the mantle output. This study complies with Neoarchean intraoceanic accretionary cycle plate tectonics in WDC.

Cementation Characteristics and Their Effect on Quality of the Upper Triassic, the Lower Cretaceous, and the Upper Cretaceous Sandstone Reservoirs, Euphrates Graben, Syria

Ibrahem Yousef, Vladimir Morozov, Vladislav Sudakov, Ilyas Idrisov

2021, 32(6): 1545-1562. doi: 10.1007/s12583-020-1065-8

[Abstract](284) [FullText HTML] (214) [PDF 33921KB](64)

Abstract:
This article presents the results of cementation characteristics and their effect on sandstone reservoir quality of the Upper Triassic Mulussa F, the Lower Cretaceous Lower Rutbah, and the Upper Cretaceous Post Judea Sandstone formations in selected fields in the Euphrates Graben area, Syria. This study emphasises the role of cementation in the evaluation of the diagenetic history of the sediments, developing effective porosity, as well as evaluation of reservoirs stimulation procedures and potential for formation damage of the sandstone reservoirs. Quartz cement is present as well developed tabular or pyramidal syntaxial overgrowths. Kaolinite cement is present as vermicular aggregates which are most abundant within sandstones of the Mulussa F Formation. Carbonate cements include siderite and dolomite. Four lithofacies were identified within the studied formations; lithofacies-1 and 2 correspond to fluvial depositional environments, lithofacies-3 and 4 correspond to fluvial to estuarine channel environments. The Post Judea Sandstone and the Lower Rutbah reservoir units are typically lithofacies-3 sequences in which quartz overgrowths are the dominant cement. Because the total cement is more extensive in the Post Judea Sandstone Formation than in the Lower Rutbah Formation, resulting in high porosity (up to 26%) and permeability (6 000 mD), the reservoir quality is predicted to be best in the Post Judea Sandstone Formation. The reservoir units in the Mulussa F Formation contain the highest cement volumes comprised of early siderite and kaolinite, which, with the development of compaction-resisting quartz overgrowths and resultant compactional pore volume loss, has resulted in typically lower porosity being preserved than in the Lower Rutbah and Post Judea Sandstone formations.

Glacier-Glacial Lake Interactions and Glacial Lake Development in the Central Himalaya, India (1994-2017)

Pratima Pandey, Sheikh Nawaz Ali, Prashant Kumar Champati Ray

2021, 32(6): 1563-1574. doi: 10.1007/s12583-020-1056-9

[Abstract](217) [FullText HTML] (168) [PDF 13570KB](54)

Abstract:
Despite several regional glacier and glacier lake inventories, the relationship between receding glacier, glacial lake evolution (glacial-lake interactions) and their sensitivity to different forcing factors have not been properly understood yet. To better understand these processes, we used satellite images collected in 1994, 2015 and 2017 to monitor the spatially-explicit evolution of glacial lakes and glacier changes. The results show a total of 1 353 glacial lakes covering an area of 7.96 km² in the year 2015. Out of these, a total of 137 glacial lakes having an area of > 0.01 km² and located within 2 km periphery of mother glacier have been selected for the monitoring of spatial development between 1994 and 2017. We found an increase in the total lake area from ~4.9 to ~7.73 km² between 1994 and 2017, corresponding to an overall expansion of ~57%. The total area covered by the glaciers associated with these lakes reduced from ~365 km² in 1994 to ~358 km² in 2017, accounting for a glacier loss of ~7 km² and corresponding to ~1.92% reduction. Our study results are in agreement with global glacier behavior, revealing a rapid glacier recession and accelerated glacial lake expansion under an unprecedented climate change scenario. In addition, the results suggest a significant reduction in the glacier area and a close relationship between the glacier melting and lake changes.

Glacial Lakes in the Andes under a Changing Climate: A Review

Bijeesh Kozhikkodan Veettil, Ulrich Kamp

2021, 32(6): 1575-1593. doi: 10.1007/s12583-020-1118-z

[Abstract](354) [FullText HTML] (282) [PDF 21131KB](72)

Abstract:
In this article, we review the current knowledge of the glacial recession and related glacial lake development in the Andes of South America. Since the mid-1980s, hundreds of glacial lakes either expanded or formed, and predictions show that additional hundreds of lakes will form throughout the 21st century. However, studies on glacial lakes in the Andes are still relatively rare. Many glacial lakes pose a potential hazard to local communities, but glacial lake outburst floods (GLOFs) are understudied. We provide an overview on hazards from glacial lakes such as GLOFs and water pollution, and their monitoring approaches. In real-time monitoring, the use of unmanned aerial systems (UASs) and early warning systems (EWSs) is still extremely rare in the Andes, but increasingly authorities plan to install mitigation systems to reduce glacial lake risk and protect local communities. In support, we propose an international remote sensing-based observation initiative following the model of, for example, the Global Land Ice Measurements from Space (GLIMS) one, with the headquarters in one of the Andean nations.

2021 Vol. 32, No. 6

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