A 1642 Ma age for the Fraynes Formation,Birrindudu Basin,confirms correlation with the economically significant Barney Creek Formation,McArthur Basin,Northern Territory

Abstract

An interpreted CA–IDTIMS age of 1642.2?±?3.9?Ma for a volcanogenic tuffaceous siltstone from the previously undated Fraynes Formation of the Birrindudu Basin in the northwestern Northern Territory enables a rigorous chronostratigraphic correlation to be made with the economically important Barney Creek Formation of the southern McArthur Basin. This result supports previous interpretations that these geographically widely separated formations are probably linked in the subsurface. It also establishes the stratigraphic interval encompassing the Fraynes and Barney Creek formations as a potential target for greenfields base metals and petroleum exploration programs across the greater McArthur Basin.

• KEY POINTS

• A new interpreted CA–IDTIMS age provides a chronostratigraphic link between the Fraynes Formation of the Birrindudu Basin and the economically important Barney Creek Formation of the southern McArthur Basin.

• The Fraynes and Barney Creek formations are probably linked in the subsurface.

• This stratigraphic level is a potential target for greenfields base metals and petroleum exploration across the greater McArthur Basin.

     

The Santa Catarina Granulite Complex of Southern Brazil: A Review

The Santa Catarina granulite complex (SCGC) is mainly composed of intermediate to felsic orthogneisses of trondhjemitic, tonalitic and granodioritic compositions (TTG) intruded by subsidiary basic-ultrabasic rocks, and a small metasedimentary component. The ortho-derived rocks belong to different igneous suites. Nd model ages TDM of 2.7-2.8 Ga date the first major mantle-crust differentiation in the area, while 2.6 Ga Rb-Sr whole rock isochron ages in both depleted and undepleted granulites and Nd model ages of 2.4-2.3 Ga record subsequent magmatic events. Regional granulite-facies which reached 800°C at ca. 5-8 kb finished at 2.3 Ga according to U-Pb zircon and Sm-Nd mineral isochron ages, and was also followed by a regional amphibolite facies metamorphism at 2.0 Ga, recorded by Pb-Pb, Rb-Sr whole rock isochrons in rocks with little orthopyroxene and predominant amphibole. Tectonic stabilization was complete at the end of the Transamazonian Cycle at 1.9 Ga as indicated by K-Ar mineral ages, and apart from local reactivation along faults at the margins of the complex and granite intrusion, the SCGC was not affected by the Neoproterozoic Brasiliano Cycle, during which it acted as a microplate and was incorporated onto the eastern border of the Paraná Craton. Isotopic and geochemical characteristics of Brasiliano crustal granites which intrude the complex suggest that it is not an adequate source for the melts, and thus the SCGC may be underlain by different Paleoproterozoic (2.0 Ga) crustal material.     

Geochemistry and zircon geochronology of paleoproterozoic granitoids: Further evidence on the magmatic and crustal evolution of the São Luís cratonic fragment, Brazil

The Tromaí Intrusive Suite is the predominant exposed unit of the São Luís cratonic fragment in northern Brazil. The suite forms batholiths and stocks of granitoids that were emplaced between 2168 ± 4 Ma and 2149 ± 4 Ma and intruded a 2240 ± 5 Ma old metavolcano-sedimentary sequence. The batholiths are composed of a variety of petrographic types that have been grouped in three sub-units, based on the predominant petrographic type, and named Cavala Tonalite, Bom Jesus Granodiorite, and Areal Granite, from the more primitive to the more evolved phases, in addition to subordinate shallow felsic intrusions. The Tromaí Suite is an expanded magmatic association comprising minor mafic rocks to predominantly intermediate and felsic, low- to high-K, and metaluminous to weakly peraluminous granitoids that follow a Na-enriched calc-alkaline trend. Combined rock association, geochronology, Nd isotopes, and geochemical signature indicate that the Tromaí Suite formed from magmas derived from juvenile protoliths modified by fractional crystallization. The juvenile protoliths included ocean plate, mantle wedge, and minor sediments. The data also indicate an intra-oceanic arc setting that possibly transitioned to a continental margin and that the Tromaí Intrusive Suite records the main accretionary stage of the Rhyacian orogen (ca. 2.24–2.15 Ma) that culminated with a collision stage at about 2.1 Ga and gave rise to the present day São Luís cratonic fragment. This time interval is coincident with the main period of crustal growth in the South American Platform and in the Paleoproterozoic terranes of the West African Craton. The beginning of this period is also coincident with the end of a period in which only minor amounts of juvenile crust is found worldwide.The Negra Velha Granite is a distinct unit that forms a few stocks that intruded the granitoids of the Tromaí Suite between 2076 and 2056 Ma ago. Negra Velha is an association of monzogranite and subordinate quartz–monzonite and syenogranite with an alkaline signature that shows high Rb–Sr–Ba enrichments, resembling shoshonitic associations. This granite represents the post-orogenic phase of the Rhyacian orogenesis.     

Structure and emplacement of granite plutons in the Paleoproterozoic crust of Eastern Burkina Faso: rheological implications

The Fada N′Gourma area in Burkina Faso is underlain by Paleoproterozoic rocks that make the northeastern West-African Craton. This region is composed of NE-trending volcano-sedimentary belts and foliated tonalites, affected by several shear zones. A generation of younger, ∼2100 Ma-old, non-foliated biotite-bearing granites intrudes the former rock units. We have investigated the younger granite pluton of Kouare that was previously considered as forming a single body with the pluton of Satenga to the west, a pluton which likely belongs to the ∼20 Ma more recent Tenkodogo-Yamba batholith. Magnetic fabric measurements have been combined with microstructural observations and the analysis of field and aeromagnetic data. The granite encloses angular enclaves of the host tonalites. Magmatic microstructures are preserved inside the pluton and solid-state, high-temperature deformation features are ubiquitous at its periphery. The presence of steeply plunging lineations in the pluton of Kouare and its adjacent host-rocks suggests that large volumes of granitic magmas became crystallized while they were ascending through the crust that was softened and steepened close to the contact. Around Kouare, the foliation in the host tonalites conforms with a map-scale, Z-shaped fold in between NNE-trending shear zones, implying a bulk clockwise rotation of the material contained in-between the shear zones, including the emplacing pluton. Regionally, the Fada N′Gourma area is concluded to result from NW-shortening associated with transcurrent shearing and vertical transfer of granitic magmas. This study concludes that the ∼2200 Myears old juvenile crust of Burkina Faso was brittle before the intrusion of the biotite-granites, became softened close to them and that gravity-driven and regional scale wrench tectonics were active together.     

Zircon U-Pb Age, Trace Element, and Hf Isotope Evidence for Paleoproterozoic Granulite-Facies Metamorphism and Archean Crustal Remnant in the Dabie Orogen

Zircon U-Pb age, trace elements, and Hf isotopes were determined for granulite and gneiss at Huaugtuling (黄土岭), which is hosted by ultrahigh-pressure metamorphic rocks in the Dabie(大别) orogen, east-central China. Cathodolumineseence (CL) images reveal core-rim structure for most zircons in the granulite. The cores show oscillatory zoning, relatively high Th/U and 176 Lu/177 Hf ratios, and high rare earth element (HREE)-enriched pattern, consistent with magmatic origin. They gave a weighted mean 207 Pb/206 Pb age of (2 766±9) Ma, dating magma emplacement of protolith. The rims are characterized by sector ur planar zoning, low Th/U and 176 Lu/177 Hf ratios, negative Euanomalies and flat HREE patterns, consistent with their formation under granulite-facies metamorphicconditions. Zircon U-Pb dating yields an age of (2 029±13) Ma, which is interpreted as a record ofmetamorphic event during the assembly of the supercontinent Columbia. The gneiss has a protolith ageof (1982±14) Ma, which is similar to the zircon U-Pb age for the granulite-facies metamorphism,suggesting complementary processes to granulite-facies metamorphism and partial melting. A fewinherited cores with igneous characteristics have 207 pb/206 Pb ages of approximately 3.53, 3.24, and 2.90Ga, respectively, suggesting the presence of Mesoarchean to Paleoarchean crustal remnants. A fewTriassic and Cretaceous metamorphic ages were obtained, suggesting the influences by the Triassiccontinental collision and postcollisional collapse in response to the Cretaceous extension. Comparingwith abundant occurrence of Triassic metamorphic zircons in ultrahigh-pressure eclogite and granitehydrous melt is evident for zircon growth in theHuangtuling granulite and gneiss during thecontinental collision. The magmatic protolithzircons from the granulite show a large variationin 176 Hf/177 Hf ratios from 0.280 809 to 0.281 289,corresponding to era(t) values of-7.3 to 6.3 andHf model ages of 2.74 to 3.34 Ga. The 2.90 Gainherited zircons show the similar Hf isotope features. These indicate that both growth of juvenile crust and reworking of ancient crust took place at the time of zircon formation. It is inferred that the Archean basement of the Yangtze block occurs in thenorth as the Dabie orogen, with ca. 2.90-2.95 Ga and 2.75-2.80 Ga as two major episodes of crustalformation.     

华北克拉通南缘古元古代末(～1.84Ga)垣头A-型花岗岩成因及其构造意义

在华北克拉通陆续发现有大量的古元古代末-中元古代早期A-型花岗岩,它们具有独特的地球化学特征和特定的形成条件,可为进一步了解A-型花岗岩的岩石成因以及华北克拉通由结晶基底的形成向稳定盖层发育时期构造环境的转折提供重要依据。本文对华北克拉通南缘产出的垣头花岗岩体进行了岩石学、锆石U-Pb年代学、元素地球化学和Nd-Hf-O同位素研究。结果表明,垣头岩体岩石类型主要为正长花岗岩,形成时代为1841±4Ma;全岩样品具有较高的铁(FeO~T/(FeO~T+MgO)=0. 75～0. 86)、富碱(K_2O+Na_2O=8. 11%～9. 13%,K_2O/Na_2O 1)、低MgO (0. 28%～0. 56%)、P_2O_5(0. 04%～0. 08%)和MnO(0. 03%～0. 04%),富含大离子亲石元素(如Rb、Th、U、K)、亏损高场强元素(Nb、Ta、Zr、Hf),具有Eu、Sr负异常,高10,000Ga/Al比值( 2. 76)、准铝质-弱过铝质且不含碱性暗色矿物,属于铝质A-型花岗岩;全岩εNd(t)=-4. 68、tC DM=2. 70Ga,锆石εHf(t)=-12. 0～-4. 71、tC8DM=3. 22～2. 78Ga,锆石δ1O=5. 4‰～6. 5‰(加权平均值为6. 1‰),指示它们是古老基底物质部分熔融的产物。结合区域上的地质特征和研究资料,我们认为垣头A-型花岗岩体形成于碰撞后的伸展环境,标志着华北克拉通1. 85Ga前后的造山运动的结束。     

Paleoproterozoic Granitoids on Liaodong Peninsula,North China Craton

Paleoproterozoic granitoids are an important constituent of the Jiao–Liao–Ji Belt(JLJB). The spatial-temporal distribution and types of Paleoproterozoic granitoids are closely related to the evolution of the JLJB. In this paper, we review the field occurrence, petrography, geochronology, and geochemistry of Paleoproterozoic granitoids on Liaodong Peninsula, northeast China. The Paleoproterozoic granitoids can be divided into pre-tectonic(～2.15 Ga; peak age=2.18 Ga) and post-tectonic(～1.85 Ga) granitoids. The pre-tectonic granitoids are magnetite and hornblende–biotite monzogranites and granodiorites. Pre-tectonic monzogranites are widespread in the JLJB and have A_2-type affinities. In contrast, pretectonic granodiorites are only present in the Simenzi area and have adakitic affinities. The post-tectonic granitoids consist of porphyritic monzogranite, syenite, diorite, granodiorite, quartz monzonite, monzogranite, and granitic pegmatite, which are adakitic rocks and I-, S-, and A_2-type granitoids. The assemblage of pre-tectonic A_2-type granitoids and adakitic rocks indicates the initial tectonic setting of the JLJB was a continental back-arc basin. The assemblage of post-tectonic adakitic rocks and I-, S-, and A_2-type granitoids indicates a post-collisional setting. The 2.20–2.15 Ga A_2-type granitoids and adakitic rocks were associated with the initial stage of back-arc extension, and the peak of back-arc extension is inferred from the subsequent(2.15–2.10 Ga) mafic intrusive activity. The ～1.90 Ga adakitic rocks mark the beginning of the postcollisional stage, which was followed by the intrusion of low-temperature S-and I-type granitoids. High-to low-pressure granitoids(S-type) were generated during the peak of post-collisional lithospheric delamination and asthenospheric upwelling. The emplacement of later granitic pegmatites occurred during the waning of the orogeny.     

Archean-Paleoproterozoic Lithospheric Mantle at the Northern Margin of the North China Craton Represented by Tectonically Exhumed Peridotites

Tectonically emplaced peridotites from North Hebei Province, North China Craton, have retained an original harzburgite mineral assemblage of olivine(54%–58%) + orthopyroxene(40%–46%)+minor clinopyroxene(1%)+spinel. Samples with boninite-like chemical compositions also coexist with these peridotites. The spinels within the peridotites have high-Al end-members with Al_2O_3 content of 30 wt % –50 wt %, typical of mantle spinels. When compared with experimentally determined melt extraction trajectories, the harzburgites display a high degree of melting and enrichment of SiO_2, which is typical of cratonic mantle peridotites. The peridotites display variably enriched light rare earth elements(REEs), relatively depleted middle REEs and weakly fractionated heavy REEs, which suggest a melt extraction of over 25% in the spinel stability field. The occurrence of arc-and SSZ-type chromian spinels in the peridotites suggests that melt extraction and metasomatism occurred mostly in a subduction-related setting. This is also supported by the geochemical data of the coexisting boninite-like samples. The peridotites have ~(187)Os/~(188)Os ratios ranging from 0.113–0.122, which is typical of cratonic lithospheric mantle. These ~(187)Os/~(188)Os ratios yield model melt extraction ages(TRD) ranging from 981 Ma to 2054 Ma, which may represent the minimum estimation of the melt extraction age. The Al_2O_3-~(187)Os/~(188)Os-proxy isochron ages of 2.4 Ga–2.7 Ga suggest a mantle melt depletion age between the Late Achaean and Early Paleoproterozoic. Both the peridotites and boninite-like rocks are therefore interpreted as tectonically exhumed continental lithospheric mantle of the North China Craton, which has experienced mantle melt depletion and subduction-related mantle metasomatism during the Neoarchean-Paleoproterozoic.     

The age and petrology of the Chimbadzi Hill Intrusion, NW Zimbabwe: first evidence for early Paleoproterozoic magmatism in Zimbabwe

The mafic-ultramafic Chimbadzi Hill intrusion in the NW of the Zimbabwe craton is a dyke with inward-dipping margins comprising magnetite peridotite, troctolite and magnetite melatroctolite. The magnetite peridotite is composed of about equal amounts of V- and Ti-bearing magnetite and olivine (Fo₆₀). The troctolite is composed of about 50% olivine (Fo_50-54), 40% plagioclase (An_53-58), 7% clinopyroxene and minor apatite and magnetite with ilmenite lamellae. Geochemical trends suggest that the Chimbadzi Hill Intrusion formed by fractional crystallisation from a single initial magma. However, the more primitive magnetite peridotite overlies the more evolved troctolite in the intrusion. This ‘apparent’ inverted stratigraphy may be due to emptying of a fractionated magma chamber from the top, or to floor subsidence during intrusion.U–Pb dating on baddeleyite reveals that the age of the Chimbadzi Hill Intrusion is 2262 ± 2 Ma. This age does not correspond to any known tectono-thermal event in the Zimbabwe Craton or adjacent metamorphic belts. It is 300 Ma younger than the late Archean Great Dyke, and 230 Ma older than other Paleoproterozoic events in and around the craton. Therefore, it may represent a so far undocumented very early Proterozoic igneous event in the Zimbabwe Craton. The intrusion represents a vanadium resource for Zimbabwe, with titanium potentially being mined as by-product.