Neoproterozoic/Lower Palaeozoic Events in Central Dronning Maud Land (East Antarctica)

New geochronological data indicate that central Dronning Maud Land in East Antarctica underwent polyphase Neoproterozoic/Lower Palaeozoic metamorphism that can be correlated with the final amalgamation of E- and W-Gondwana. Central Dronning Maud Land most probably represents part of the southern continuation of the Mozambique Belt into E-Antarctica. The Neoproterozoic/Lower Palaeozoic metamorphism is preceded by a period of anorogenic anorthosite-charnockite magmatism at ca. 600 Ma. Polyphase metamorphism is recorded from ca. 580 to 515 Ma. Voluminous syntectonic magmatism has been documented at ca. 530 Ma, which is probably the most voluminous Neoproterozoic/Lower Palaeozoic syntectonic magmatism so far recorded in E-Antarctica. The Neoproterozoic/Lower Palaeozoic structural evolution evolved in an overall sinistral transpressional setting, and thus can be correlated with the broad tectonic setting of the Mozambique Belt in Africa.     

Palaeomagnetic results from the ca. 1130 Ma Borgmassivet intrusions in the Ahlmannryggen region of Dronning Maud Land, Antarctica, and tectonic implications

Detailed palaeomagnetic and rock magnetic analyses provide improved palaeomagnetic results from 23 sites in the Borgmassivet intrusions in the Ahlmannryggen region of Dronning Maud Land, East Antarctica. These intrusions are of similar age to their host, the ca. 1130 Ma Ritscherflya Supergroup (RSG). A mean direction of D=235.4°, I=−7.6° with k=45.9 and α₉₅=4.5° was obtained from this study. When combined with previously reported results from 11 sites in the same region, including sites from the Ritscherflya Supergroup, it gives an overall mean direction for 34 sites from the igneous suite with D=236.5°, I=−3.6°, k=27.9 and α₉₅=4.8°. Isothermal remanent magnetization (IRM) experiments on several specimens suggest magnetite or titanomagnetite as the primary remanence carrier, while high temperature magnetic susceptibility experiments indicate the presence of single domain particles. These observations, together with field evidence and the high coercivities and unblocking temperatures, support a primary origin for the observed characteristic remanence. The Borgmassivet palaeomagnetic pole lies at 54.5°E, 8.3°N with A₉₅=3.3°. If Antarctica is moved to its Gondwanan position adjacent to southeast Africa, the Borgmassivet pole (BM) coincides with that of the African well-established, well-dated (1100 Ma) Umkondo Large Igneous Province pole, supporting the hypothesis that the Grunehogna craton of Dronning Maud Land was part of the Kalahari craton of southern Africa at ca. 1100 Ma.     

Palaeomagnetic constraints on the position of the Kalahari craton in Rodinia

A comparison of late Mesoproterozoic palaeomagnetic poles from the Kalahari craton and its correlative Grunehogna craton in East Antarctica shows that the Kalahari–Grunehogna craton straddled the palaeo-Equator and underwent no azimuthal rotation between ca. 1130 and 1105 Ma. Comparison of the Kalahari palaeopoles with the Laurentia APWP between 1130 and 1000 Ma shows that there was a latitudinal separation of 30±14° between Kalahari and the Llano–West Texas margin of Laurentia at ca. 1105 Ma. The Kalahari craton could have converged with southwestern Laurentia between 1060 and 1030 Ma to become part of Rodinia by 1000 Ma. In Rodinia, the Kalahari craton lay near East Antarctica with the Namaqua–Natal orogenic belt facing outboard and away from the Laurentian craton.     

New aeromagnetic view of the geological fabric of southern Dronning Maud Land and Coats Land,East Antarctica

The ice shield of Antarctica, which measures several kilometers in thickness, presents a challenge when attempting to unravel the subglacial geology. Here, we report about systematic airborne magnetic surveys conducted over the last decade, which investigated a significant part of Dronning Maud Land (DML), imaging for the first time the crustal architecture of the interior of this sector of East Antarctica. High-resolution data reveal parallel, elongated magnetic anomalies in southeastern DML. These NW–SE trending anomalies can be traced farther east into sparser Russian magnetic data sets. Several high amplitude magnetic anomalies with values above 400 nT have been observed in southwestern DML and Coats Land. They differ clearly in wavelength and amplitudes from the magnetic pattern found in the east and do not show any evidence of a Pan-African orogenic belt or suture zone connecting the Shackleton Range with eastern DML, as hypothesized in several studies. This leads to the assumption of the existence of a hitherto unrecognized large tectonic province in southeastern DML. Whereas an over 100 km long magnetic lineament in the interior of the Dronning Maud Land may reflect a major shear zone akin to the Pan-African age Heimefrontfjella shear zone. Both findings bring new evidences to the still open question about the amalgation of East and West Gondwana. In addition, the magnetic data allow mapping the eastern extent of the presumable cratonic province of Coats Land, a region considered as a key piercing point for reconstructions of Rodinia. Furthermore, the Beattie Magnetic Anomaly in southern Africa is assumed to continue into East Antarctica. Two magnetic highs in western DML are identified as possible eastward continuation of this prominent anomaly.     

Central Antarctic provenance of Permian sandstones in Dronning Maud Land and the Karoo Basin: Integration of U-Pb and TDM ages and host-rock affinity from detrital zircons

In conjugate SE Africa and Antarctica, Early Permian sandstones of the Swartrant Formation of the Ellisras Basin, Vryheid Formation of the Karoo Basin, and Amelang Plateau Formation of Dronning Maud Land (DML) were deposited after Gondwanan glaciation on a westward paleoslope. We analysed detrital zircons for U-Pb ages by a laser ablation microprobe-inductively coupled plasma mass spectrometer (LAM-ICPMS) and attached age significance only to clusters of three or more overlapping analyses. We analysed Hf-isotope compositions by a multi-collector spectrometer (LAM-MC-ICPMS) and trace elements by electron microprobe (EMP) and ICPMS. These analyses indicate the rock type and source (whether crustal or juvenile mantle) of the host magma, and a “crustal” model age (T_DM^C). The integrated analysis gives a more distinctive, and more easily interpreted, picture of crustal evolution in the provenance area than age data alone.Zircons from the Ellisras Basin are aged 2700-2540 Ma with minor populations about 2815 Ma and 2040 Ma, which correspond with the ages of the upslope parts of the proximal Kaapvaal Craton and Limpopo Belt. Mafic rock is the dominant host rock, and it reflects the Archean granite-greenstone terrane of the Kaapvaal Craton.The three Karoo Basin samples and the two DML samples have zircons with these common properties: (1) 1160-880 Ma, host magma mafic granitoid (< 65% SiO₂) derived from juvenile depleted mantle sources (ε_Hf positive) at 1.65 Ga and 1.35 Ga, with T_DM^C of 2.0-0.9 Ga; (2) 760 to 480 Ma, host magma granitoid and low-heavy rare earth element rock (?alkaline rock-carbonatite), derived from mixed crustal and juvenile depleted mantle sources (ε_Hf positive and negative) at 1.50 Ga and 1.35 Ga, with T_DM^C of 2.0-0.9 Ga.Together with similar detrital zircons in Triassic sandstone of SE Australia, these properties reflect those in upslope central Antarctica, indicating a provenance of ∼ 1000 Ma (Grenville) cratons embedded in 700-500 Ma (Pan-Gondwanaland) fold belts. Detrital zircons in Cambrian sediments of the Ellsworth-Whitmore Mountains block and Cambrian metasediments of the Welch Mountains with comparable properties suggest that the central Antarctic provenance operated also in the ∼ 500 Ma Cambrian.     

Metamorphic evolution of the Maud Belt: P–T–t path for high-grade gneisses in Gjelsvikfjella, Dronning Maud Land, East Antarctica

A metamorphic petrological study, in conjunction with recent precise geochronometric data, revealed a complex P–T–t path for high-grade gneisses in a hitherto poorly understood sector of the Mesoproterozoic Maud Belt in East Antarctica. The Maud Belt is an extensive high-grade, polydeformed, metamorphic belt, which records two significant tectono-thermal episodes, once towards the end of the Mesoproterozoic and again towards the late Neoproterozoic/Cambrian. In contrast to previous models, most of the metamorphic mineral assemblages are related to a Pan-African tectono-thermal overprint, with only very few relics of late Mesoproterozoic granulite-facies mineral assemblages (M₁) left in strain-protected domains. Petrological and mineral chemical evidence indicates a clockwise P–T–t path for the Pan-African orogeny. Peak metamorphic (M_2b) conditions recorded by most rocks in the area (T = 709–785 °C and P = 7.0–9.5 kbar) during the Pan-African orogeny were attained subsequent to decompression from probably eclogite-facies metamorphic conditions (M_2a).The new data acquired in this study, together with recent geochronological and geochemical data, permit the development of a geodynamic model for the Maud Belt that involves volcanic arc formation during the late Mesoproterozoic followed by extension at 1100 Ma and subsequent high-grade tectono-thermal reworking once during continent–continent collision at the end of the Mesoproterozoic (M₁; 1090–1030 Ma) and again during the Pan-African orogeny (M_2a, M_2b) between 565 and 530 Ma. Post-peak metamorphic K-metasomatism under amphibolite-facies conditions (M_2c) followed and is ascribed to post-orogenic bimodal magmatism between 500 and 480 Ma.     

Stenian A-type granitoids in the Namaqua-Natal Belt,southern Africa,Maud Belt,Antarctica and Nampula Terrane,Mozambique:Rodinia and Gondwana amalgamation implications

We carried out SHRIMP zircon U-Pb dating on A-type granitic intrusions from the Namaqua-Natal Province,South Africa,Sverdrupfjella,western Dronning Maud Land,Antarctica and the Nampula Province of northern Mozambique.Zircon grains in these granitic rocks are typically elongated and oscillatory zoned,suggesting magmatic origins.Zircons from the granitoid intrusions analyzed in this study suggest~1025-1100 Ma ages,which confirm widespread Mesoproterozoic A-type granitic magmatism in the Namaqua-Natal(South Africa),Maud(Antarctica) and Mozambique metamorphic terrains.No older inherited(e.g.,~2500 Ma Achean basement or~1200 Ma island are magmatism in northern Natal)zircon grains were seen.Four plutons from the Natal Belt(Mvoti Pluton,Glendale Pluton,Kwalembe Pluton,Ntimbankulu Pluton) display 1050-1040 Ma ages,whereas the Nthlimbitwa Pluton in northern Natal indicates older 1090-1080 Ma ages.A sample from Sverdrupfjella,Antarctica has~1091 Ma old zircons along with~530 Ma metamorphic rims.Similarly,four samples analysed from the Nampula Province of Mozambique suggest crystallization ages of~1060-1090 Ma but also show significant discordance with two samples showing younger~550 Ma overgrowths.None of the Natal samples show any younger overgrowths.A single sample from southwestern Namaqualand yielded an age of~1033 Ma.Currently available chronological data suggest magmatism took place in the Namaqua-Natal-MaudMozambique(NNMM) belt between~1025 Ma and~1100 Ma with two broad phases between~1060-1020 Ma and 1100-1070 Ma respectively,with peaks at between~1030-1040 Ma and~1070-1090 Ma.The age data from the granitic intrusions from Namaqualand.combined with those from Natal,Antarctica and Mozambique suggest a crude spatial-age relationship with the older1070 Ma ages being largely restricted close to the eastern and western margins of the Kalahari Craton in northern Natal,Mozambique.Namaqualand and WDML Antarctica whereas the younger 1060 Ma ages dominate in southern Natal and western Namaqualand and are largely restricted to the southern and possibly the western margins of the Kalahari Craton.The older ages of magmatism partially overlap with or are marginally younger than the intracratonic Mkondo Large lgneous Provinee intruded into or extruded onto the Kalahari Craton,suggesting a tectonic relationship with the Maud Belt.Similar ages from granitic augen gneisses in Sri Lanka suggest a continuous belt stretching from Namaqualand to Sri Lanka in a reconstituted Gondwana,formed during the terminal stages of amalgamation of Rodinia and predating the East African Orogen.This contiguity contributes to defining the extent of Rodinia-age crustal blocks,subsequently fragmented by the dispersal of Rodinia and Gondwana.     

Petrology and geochemistry of the Grubergebirge anorthosite and marginal rocks,central Dronning Maud Land: Further characterization of the Late Neoproterozoic magmatic event in East Antarctica

The Grubergebirge anorthosite, a Late Neoproterozoic massif-type anorthosite, was emplaced into Late Mesoproterozoic-aged metamorphosed orthogneisses and supracrustal rocks in the Wohlthat Mountains. Mineralogically and chemically, the marginal rocks to the anorthosite massif classify as ferromonzodiorite and ferromonzonite. Variations in trace and rare earth element abundances and normalized patterns between the anorthosite and associated marginal ferromonzodiorite (and minor ferromonzonite) and distinct differences between the ferromonzodiorite and ferromonzonite have been observed. Whereas the magmas from which the anorthosite crystallized have been slightly contaminated by incorporating crustal material (now occurring as enclaves), the marginal ferromonzodiorite represent rocks that originated due to mixing (hybridization) of injected primitive ferrodiorite magma(s) with preexisting crustal material, resulting in the hybrid ferromonzodiorite.     

Evidence for constitutional supercooling from orthopyroxene - pigeonite - plagioclase relations in the Borgmassivet suite of western Dronning Maud Land,Antarctica

Summary The Kullen-type sills of western Dronning Maud land are up to 400 m thick and form part of the Meso- to Neoproterozoic Borgmassivet suite. They consist of a Basal, Cumulate, Central and Upper zone. The petrography of the Cumulate zone, with special emphasis on the occurrences at Hammer Heads in the Annandagstoppane area, is described. Microstructures, such as delicate chains of orthopyroxene primocrysts and an increase in grain size of plagioclase chadacrysts from the cores to the margins of postcumulus clinopyroxene oikocrysts, indicatein situ crystallization. Composite grains with orthopyroxene at the core, which is mantled, in turn, by inverted pigeonite, a second generation orthopyroxene and, on the outer margin, clinopyroxene in a reaction relation with the orthopyroxene, are sporadically developed.The relation between the composite structures and plagioclase nucleation and growth is investigated by the study of plagioclase grain size distributions, nucleation density and An compositions of plagioclase cores, which indicate that two periods of plagioclase nucleation occurred. The first period of nucleation, represented by plagioclase chadacrysts enclosed in primocrysts of orthopyroxene, occurred during supercooling following intrusion. Release of latent heat of crystallization resulted in an increase in magma temperature and a normal sequence of fractional crystallization in which orthopyroxene crystallization was succeeded by pigeonite. The second period of plagioclase nucleation depleted the interface liquid in the zone of crystallization of calcium. The composition of the interface liquid was thus changed from the compositional field from which pigeonite crystallized to that from which orthopyroxene crystallized (cf the liquidus boundaries in the system CaMgSi₂O₆-Mg₂SiO₄SiO₂ ofKushiro, 1972). This constitutional supercooling (Chalmers, 1964;Tiller, 1991) resulted in crystallization of orthopyroxene and the reverse crystallization sequence En-rich orthopyroxene -pigeonite - En-rich orthopyroxene, prior to reaction of the last-formed orthopyroxene with the liquid to form clinopyroxene and subsequent inversion of pigeonite. A possible crystallization sequence is discussed by making use of thespeculative Fo-Di-An-SiO₂ (forsterite-diopside-anorthite-silica) system ofMorse (1980).

---

Orthopyroxen-Pigeonit-Plagioklas-Beziehungen in der Borgmassivet Suite des westlichen Dronning Maud Landes, Antarktis: Hinweise auf konstitutionelles Supercooling

Zusammenfassung Lagergänge des Kullen-Types im westlichen Dronning Maud Land sind bis zu 400m mächtig und sind Teil der meso- bis neoproterozoischen Borgmassivet-Abfolge. Sie bestehen aus einer basalen, einer Cumulat, einer zentralen und einer oberen Zone. Die Petrographie der Cumulat-Zone wird mit besonderer Betonung der Vorkommen von Hammer Heads im Annandagstoppane-Gebiet beschrieben. Mikrostrukturen, wie z.B. feinkörnige Ketten von Orthopyroxen-Primokristen und eine Zunahme in der Korngröße von Plagioklas-Chadakristen von den Kernen zu den Rändern in postcumulus Klinopyroxen-Oikokristen, weisen aufin situ Kristallisation hin. Zusammengesetzte Körner mit Orthopyroxen im Kern, der von invertiertem Pigeonit umgeben ist, einer zweiten Generation Orthopyroxen, und, am äußeren Rand, Klinopyroxen in Reaktionsbeziehung mit Orthopyroxen, kommen sporadisch vor.Die Beziehung zwischen diesen zusammengesetzten Strukturen und der Nukleation, sowie dem Wachstum von Plagioklas wurde durch das Studium der Plagioklaskorngrößen, der Nukleationsdichte und der An-Zusammensetzung von Plagioklas-Kernen untersucht. Diese weisen darauf hin, daß zwei Perioden von Plagioklas-Nukleation vorgekommen sind. Die erste Nukleationsperiode wird durch Plagioklas-Chadakristen, die in Primokristen von Orthopyroxen eingeschlossen sind repräsentiert, und fand während einer Phase von Supercooling im Anschluß an die Intrusion statt. Das Freiwerden latenter Kristallisationswärme führte zu einer Zunahme der Magmentemperatur und zu einer normalen Abfolge fraktionierter Kristallisation, in der auf die Orthopyroxen-Kristallisation Pigeonit folgte. Die zweite Phase der Plagioklasnukleation führte zu einer Verarmung der Schmelze an Kalzium in der Kristallisationszone. Die Zusammensetzung der Schmelze bewegte sich so aus dem Pigeonitfeld, in das, aus dem Orthopyroxen kristallisierte (siehe die Liquidus Grenzen im System CaMgSi2O₆-Mg₂SiO₄-SO₂ von Kushiro, 1972). Dieses konstitutionelle Supercooling (Chalmers, 1964;Tiller, 1991) führte zur Kristallisation von Orthopyroxen, und der reversen Kristallisationsabfolge En-reicher Orthopyroxen-Pigeonit-En-reicher Orthopyroxen, noch vor der Reaktion des zuletzt gebildeten Orthopyroxenes mit der Schmelze, das zur Entstehung von Klinopyroxen und zur anschließenden Inversion von Pigeonit führte. Eine mögliche Kristallisationsabfolge wird auf der Basis desspekulativen Fo-Di-An-SiO₂ (Forsterit-Diopsid-Anorthit-Quartz) Systems von Morse (1980) diskutiert.

---

---

With 12 Figures     

Records of climatic changes and volcanic events in an ice core from Central Dronning Maud Land (East Antarctica) during the past century

The depth profiles of electrical conductance, δ¹⁸O,²¹⁰Pb and cosmogenic radio isotopes¹⁰Be and³⁶Cl have been measured in a 30 m ice core from east Antarctica near the Indian station, Dakshin Gangotri. Using²¹⁰Pb and δ¹⁸O, the mean annual accumulation rates have been calculated to be 20 and 21 cm of ice equivalent per year during the past ∼ 150 years. Using these acumulation rates, the volcanic event that occurred in 1815 AD, has been identified based on electrical conductance measurements. Based on δ¹⁸O measurements, the mean annual surface air temperatures (MASAT) data observed during the last 150 years indicates that the beginning of the 19th century was cooler by about 2‡ C than the recent past and the middle of 18th century. The fallout of cosmogenic radio isotope¹⁰Be compares reasonably well with those obtained on other stations (73‡ S to 90‡ S) from Antarctica and higher latitudes beyond 77‡N. The fallout of³⁶Cl calculated based on the present work agrees well with the mean global production rate estimated earlier by Lal and Peters (1967). The bomb pulse of³⁶Cl observed in Greenland is not observed in the present studies – a result which is puzzling and needs to be studied on neighbouring ice cores from the same region.     

Petrogenesis of massif-type anorthosite complex, Gruber, Central Dronning Maud Land, East Antarctica： Implications for magma source and evolution

The origin of anorthosite and associated igneous gabbronorite and ferrodiorite was investigated through detailed study of a typical massif-type anorthosite complex from Gruber, Central Dronning Maud Land, East Antarctica. Field observations showed that the Gruber Complex is made up of gabbronorite-anorthosite pluton which was intruded by ferrodiorite dykes. Systematic samples collected from the Gruber Complex revealed significant geochemical variations within the region. Four rock types have been identified, based on modal proportions of mineral phases and their geochemistry data. Clinopyroxene-gabbronorite and plagioclase-gabbronorite are the two types of gabbronorite with the dominance of clinopyroxene and plagioclase, respectively. Anorthosite is represented by rocks having predominance of plagioclase with minor clinopyroxene. Ferrodiorite is characterized by modal abundance of orthopyroxene and Fe-Ti oxide. Major and trace element systematics showed that all the four rock types are co-magmatic and are related through fractional crystallization. Based on this study, it is reported that clinopyroxene was the first phase to crystallize followed by plagioclase and then Fe-Ti oxides. Furthermore, trace element composition of the parental melt was calculated using LA-ICPMS analysis of the most primitive, pure clinopyroxene found in the clinopyroxene gabbronorite. Our analyses suggested that the parental melt was similar to that of continental arc basalt and showed signatures of subduction-related metasomatism. Based on mineral chemical and geochemical data, it is interpreted that the parent melt went through changing sequence of crystallization which led to the formation of massive anorthosite.     

New SHRIMP,Rb/Sr and Sm/Nd isotope and whole rock chemical data from central Mozambique and western Dronning Maud Land,Antarctica: Implications for the nature of the eastern margin of the Kalahari Craton and the amalgamation of Gondwana

Whole rock major and trace element data from granitoids adjacent to the Kalahari Craton–Mozambique–Maud Belt boundary are described. The data from ～1140 Ma old granodioritic and ～1110 Ma old granitic bodies in the Mozambique Belt show that they are typical of calc-alkaline and A-type granitoids respectively. Radiogenic Rb/Sr and Sm/Nd isotope data from the two granitoid bodies suggest significant older crustal contributions during their genesis. The granodioritic gneisses show T_DM model ages of ～2100–3500 Ma whereas megacrystic granitic gneisses have T_DM model ages of ～1600–3100 Ma. Granite from the Archaean-age Kalahari Craton has T_DM model ages of ～3000–3500 Ma.The data from Mozambique are compared with whole rock major and trace element chemistry and U/Pb zircon SHRIMP data from the Maud Belt in western Dronning Maud Land. These show that ～1140 Ma old granodioritic gneisses in Sverdrupfjella and Kirwanveggan have similar ages and chemical compositions to similar rocks in central Mozambique. Radiogenic isotope characteristics of the gneisses from central Mozambique and Sverdrupfjella are similar and suggest older crustal contributions in contrast to the juvenile nature of the gneisses from Kirwanveggan.Similarly, ～1090 Ma old granitic gneisses from central Mozambique, Sverdrupfjella and Kirwanveggan have similar ages and A-type chemical compositions. In contrast the radiogenic isotope compositions from Kirwanveggan are juvenile whereas those from central Mozambique show a significant older crustal contribution.The whole rock radiogenic isotope data can be interpreted to suggest that the Mesoproterozoic Mozambique Belt rocks were generated by partial melting which probably involved mixing of Archaean/Paleoproterozoic crust and younger Mesoproterozoic juvenile magma at ～1100 Ma and suggest that the Kalahari Craton probably extends eastwards at depths for more than 30 km from its exposure at surface.The data support correlations between the Mozambique Belt and the Maud Belt in Antarctica in general and more specifically show similarities between the Kalahari Craton boundary and the Mozambique–Maud Belt in lithologies immediately adjacent to that boundary.Two episodes of anatectic migmatisation are recognized in rocks from the Mozambique Belt in central Mozambique. These show an earlier migmatitic vein phase oriented parallel to the planar foliation in the granitic and tonalitic gneisses and a later discordant vein phase which is oriented parallel to localized but intense N–S oriented shearing along the Kalahari Craton/Mozambique Belt boundary zone. SHRIMP zircon data from the younger migmatitic vein phase suggests a crystallization age of 997 ± 4 Ma. Small numbers of inherited zircons have ages of ～2700 Ma and ～1100–1200 Ma. Younger discordant analyses suggesting metamorphic disturbance between ～400 Ma and 550 Ma are seen. The data imply the high strain along the eastern margin of the Kalahari Craton in the Manica area, occurred at ～1000 Ma and not at ～450 Ma as was previously thought. The data suggest the Pan African deformation and metamorphism in the area involved minor reworking. The undeformed to weakly deformed Tchinadzandze Granodiorite intruded into the Kalahari Craton has an age of 2617 ± 16 Ma.     

川西峨边地区金口河辉绿岩脉SHRIMP锆石U-Pb年龄及其对Rodinia裂解的启示

川西金口河地区中元古代峨边群中发育大量基性侵入岩,其具体侵位时间对厘定峨边群的时限、研究扬子西缘前寒武纪构造属性具有重要意义。对峨边群枷担桥组层型剖面中一条辉绿岩脉进行了SHRIMP锆石U-Pb定年。结果表明,该辉绿岩脉的锆石发育清楚且宽缓的内部环带结构,具有较高的Th/U比值,属典型基性岩浆成因的锆石,其206Pb/238U年龄加权平均值为(813.4±8.2)Ma(MSWD=0.56),代表了该辉绿岩脉的侵位时间,也限定了峨边群枷担桥组的上限年龄。综合扬子西缘新元古代中期基性岩浆活动的最新同位素年龄,证实存在2个主要的活动期:830~800Ma和780~745Ma,且830~800Ma期间基性岩浆活动的峰值年龄为(821±2)Ma。这与前人总结的华南和澳大利亚同期基性岩浆活动的年龄分布特征完全一致,暗示川西金口河辉绿岩脉的形成可能与新元古代导致Rodinia超大陆裂解的地幔柱幕式活动有关。     

川西峨边地区金口河辉绿岩脉SHRIMP锆石U-Pb年龄及其对Rodinia裂解的启示

川西金口河地区中元古代峨边群中发育大量基性侵入岩,其具体侵位时间对厘定峨边群的时限、研究扬子西缘前寒武纪构造属性具有重要意义。对峨边群枷担桥组层型剖面中一条辉绿岩脉进行了SHRIMP锆石U-Pb定年。结果表明,该辉绿岩脉的锆石发育清楚且宽缓的内部环带结构,具有较高的Th/U比值,属典型基性岩浆成因的锆石,其206Pb/238U年龄加权平均值为(813.4±8.2)Ma(MSWD=0.56),代表了该辉绿岩脉的侵位时间,也限定了峨边群枷担桥组的上限年龄。综合扬子西缘新元古代中期基性岩浆活动的最新同位素年龄,证实存在2个主要的活动期：830~800Ma和780~745Ma,且830~800Ma期间基性岩浆活动的峰值年龄为(821±2)Ma。这与前人总结的华南和澳大利亚同期基性岩浆活动的年龄分布特征完全一致,暗示川西金口河辉绿岩脉的形成可能与新元古代导致Rodinia超大陆裂解的地幔柱幕式活动有关。     

Humite- and scapolite-bearing assemblages in marbles and calcsilicates of Dronning Maud Land, Antarctica: new data for Gondwana reconstructions

This study investigates marbles and calcsilicates in Central Dronning Maud Land (CDML), East Antarctica. The paleogeographic positioning of CDML as part of Gondwana is still unclear; however, rock types, mineral assemblages, textures and P–T conditions observed in this study are remarkably similar to the Kerala Khondalite Belt in India. The CDML marbles and calcsilicates experienced a Pan-African granulite facies metamorphism at c. 570 Ma and an amphibolite facies retrogression at c. 520 Ma. The highest grade assemblage in marbles is forsterite+spinel+calcite+dolomite, in calcsilicates the assemblages are diopside+spinel, diopside+garnet, scapolite+wollastonite+clinopyroxene±quartz, scapolite±anorthite±calcite+clinopyroxene+wollastonite. These assemblages constrain the peak metamorphic conditions to 830±20 °C, 6.8±0.5 kbar and X _CO2>0.46. During retrogression, highly fluoric humite-group minerals (humite, clinohumite, chondrodite) replaced forsterite, and garnet rims formed at the expense of scapolite during reactions with wollastonite, calcite or clinopyroxene but without involvement of anorthite. Metamorphic conditions were about 650 °C, 4.5±0.7 kbar, 0.2< X _CO2^fluid<0.36, and the co-existence of garnet, clinopyroxene, wollastonite and quartz constrains f_O2 to FMQ-1.5 log units. Mineral textures indicate a very limited influx of H₂O-rich fluid during amphibolite facies retrogression and point to significant variations of fluid composition in mm-sized areas of the rock. Gypsum was observed in two samples; it probably replaced metamorphic anhydrite which appears to have formed under amphibolite facies conditions. The observed extensive anorogenic magmatism (anorthosites, A-type granitoids) and the character of metamorphism between 610 and 510 Ma suggest that the crustal thermal structure was characterized by a long-lived (50–100 Ma) rise of the crustal geotherm probably caused by magmatic underplating.     

冈瓦纳大陆边缘罗斯运动的岩浆活动时代:来自北维多利亚地松散沉积物锆石年龄的证据

横穿南极大陆中部的横贯南极山脉是早古生代时期古太平洋向东冈瓦纳活动大陆边缘俯冲形成的罗斯造山带,该阶段的地层沉积、变形变质以及花岗质岩浆侵入代表了罗斯运动的演化过程。由于岩浆活动与沉积地层和变形变质在时代上存在明显差异,罗斯运动的时代仍缺乏精确的限定。通过采集北维多利亚地难言岛地区冰碛物和海岸沉积物中的松散砂砾石样品,并进行碎屑锆石U-Pb测年得出:4件不同粒径的冰碛物和海岸沉积样品中的碎屑锆石年龄峰谱具有单一峰谱的特征,年龄区间为2443~323 Ma,主要集中于530~450 Ma之间,峰值年龄约为485 Ma;锆石Th/U比值均大于0.1,而且以＞0.4为主,其CL图像也具有明显的振荡环带,稀土元素特征主体具有岩浆锆石的特征,反映了样品物源区岩浆活动的时代特征。碎屑锆石年龄组成与周缘地区岩浆活动和陆内变形以及沉积地层时代基本一致,表明北维多利亚地及其周缘地区在罗斯运动晚期陆内变形阶段的岩浆活动应持续至450 Ma,这可能代表了罗斯运动结束的时代,该结果为冈瓦纳大陆边缘罗斯运动的构造演化过程提供了新的约束。      

Timing and conditions of poly-phase metamorphism within the Twelve Mile Bay shear zone: implications for the evolution of mid-crustal decollement zones and western Grenville tectonics

The Twelve Mile Bay assemblage (TMBa) forms the high-strain interior of the Twelve Mile Bay shear zone (TMBsz), a major ductile decollement zone within the western Canadian Grenville orogen. Metasupracrustal gneiss within the TMBa preserves evidence for an early granulite facies (?10–11 kbar and ?840°C) metamorphism overprinted by amphibolite facies (?5–7 kbar and ?650°C) assemblages that define the high-strain shear zone fabric. U–Pb zircon ages for TMBa samples were determined by LA-ICP-MS. A low-strain amphibolite pod with partially preserved granulite facies assemblage and textures yielded an anchored discordia intercept of 1157 ± 11 Ma and ²⁰⁷Pb/²⁰⁶Pb weighted average of 1146 ± 10 Ma. Three higher strain samples with recrystallized amphibolite facies assemblages all yield younger ages, with ²⁰⁷Pb/²⁰⁶Pb weighted averages of 1125 ± 16, 1110 ± 8, and 1095 ± 17 Ma. Phase equilibrium modelling shows that up to 40 vol.% anatectic melt could have been produced in TMBa pelitic rocks during peak metamorphic conditions, and thus, much of the package likely would have been substantially weakened during the early stages of TMBsz development. Strain apparently continued to accumulate within the TMBa until ca. 1100 Ma, concurrent with pegmatite dike emplacement and hydration along the base of the overlying interior Parry Sound domain (iPSD), perpetuating TMBsz activity during cooling and exhumation to shallower crustal levels. Similarities between the TMBa and the upper parts of the basal PSD (bPSD), in terms of timing and conditions of metamorphism and shearing, as well as structural position relative to the overlying iPSD allochthon, indicate that these units are likely correlative. The composite bPSD–TMBa system appears to have contemporaneously localized strain within the middle orogenic crust during early to middle stages of Grenvillian collision, providing a petrologically constrained mechanism for the long distance transport of mid-crustal nappes predicted in thermal-mechanical models of continental collision for this area.     

东南极拉斯曼丘陵夕线石榴二长片麻岩的格林威尔期变质作用和构造意义

东南极拉斯曼丘陵出露的中-高级变质岩是普里兹湾基底岩系的主要组成部分,是研究普里兹湾高级区组成与演化的理想对象。本文对拉斯曼丘陵米洛半岛地区的夕线石榴二长片麻岩进行了岩石学、相平衡模拟和地质年代学研究。研究结果表明拉斯曼丘陵夕线石榴二长片麻岩矿物组合为石榴子石+堇青石+斜长石+钛铁矿+夕线石+石英±黑云母±钾长石,经历了格林威尔期(~980Ma)高温变质,其峰期阶段(M1)的矿物组合为石榴子石+斜长石+钾长石+钛铁矿+夕线石+石英,变质温压条件为:~820℃、~9.6kbar。夕线石榴二长片麻岩的岩浆锆石比变质锆石具有更低的LREE和更高的HREE含量,其原岩的沉积年龄不早于~1073Ma,物源主要为中元古代岩浆岩(S-型花岗岩)。拉斯曼丘陵在格林威尔期经历了陆-陆碰撞,进而快速折返、剥蚀作用的过程。     

阿尔金且末—若羌地区新元古代花岗质岩石成因及其对Rodinia超大陆汇聚时限的约束

阿尔金造山带广泛发育新元古代花岗质岩浆岩,形成年龄为800~1000 Ma,可能是Rodinia超大陆汇聚阶段的产物,因此新元古代花岗质岩浆的研究对探讨阿尔金造山带的演化过程具有重要意义。选取南阿尔金地块且末—若羌地区的花岗质岩石为研究对象,开展了详细的岩石学、岩石地球化学及地质年代学研究。结果表明：①且末—若羌地区3类花岗质岩石的轻稀土元素相对于重稀土元素富集,显示右倾配分模式,具明显的负Eu异常（δEu=0.14~0.6）,富集Th、U、K等大离子亲石元素,亏损Ba、Ti、Nb、Ta、Sr等高场强元素;②锆石U-Pb年龄为899~915 Ma。综合区域地质演化历史表明,且末—若羌地区3类花岗质岩石形成于同碰撞构造环境,是Rodinia超大陆汇聚阶段板块之间俯冲、碰撞的产物。