Ma Amphibolites Research Articles

Paleozoic evolution of the Yukon-Tanana terrane of the North American Cordillera, NW British Columbia

Abstract The origins and primary relationships between tectono-stratigraphic units are fundamental to the terrane concept in accretionary orogens, but they are challenging to assess in metamorphic terranes. In NW British Columbia, three tectonically bounded metamorphic suites of the Yukon-Tanana terrane formed in distinct tectonic settings, based on high-spatial-resolution geochronology and immobile trace-element geochemistry. The Florence Range suite comprises late Neoproterozoic or younger to pre–latest Devonian metasedimentary rocks derived from continental crust, 360 ± 4 Ma calc-alkaline intermediate orthogneiss, and 357 ± 4 Ma amphibolite with oceanic-island basalt composition, consistent with rifting of a continental margin. The detrital signature is dominated by late Mesoproterozoic zircon, which indicates different sources than other parts of the Yukon-Tanana terrane. The Boundary Ranges suite comprises pre–Late Devonian metasedimentary rocks derived in part from a mafic source, amphibolite derived from subduction-zone metasomatized mantle, and 369 ± 4 Ma to 367 ± 7 Ma calc-alkaline felsic to intermediate orthogneiss. The Whitewater suite comprises meta-chert, graphite-rich metapelite, and amphibolite with back-arc basin basalt composition consistent with an anoxic basin near a volcanic source. Our data indicate that the Florence Range and Boundary Ranges suites were separate until at least the Early Mississippian and may have formed a composite terrane since the Permian, whereas the relationship with the Whitewater suite is uncertain. We compare the Paleozoic evolution of the Yukon-Tanana terrane in NW British Columbia with several modern analogues in the west and southwest Pacific Ocean.

The implication of two episodic Precambrian supercontinents convergence events from enriched mantle beneath the Yangtze Block: Constraints from the Zhangbaling mafic rocks

The Yangtze Block bears witness to a prolonged Precambrian evolutionary history and records the assembly of Precambrian supercontinents. Significantly, compared with the widely recognized ancient continental crustal growth and reconstruction, subduction-related mafic magmatism contains detailed possess of crust-mantle interaction in convergent plate boundaries. This paper reports Neoproterozoic mafic igneous rocks, represented by the 808–787 Ma amphibolites, from the Zhangbaling Uplift, northeastern sector of the Yangtze Block. Their prominent arc-affinity trace-element geochemical compositions with high concentration of LILE, the trough of Th, U, Zr and Hf in the spider diagram as well as natural δ18O values (5.3–6.5‰), indicate an interaction between the lithospheric mantle wedge peridotite and slab-derived fluids during early Neoproterozoic subduction event, and deficient in the involvement of felsic melts from crustal materials and sediments. Analyses of radioisotopes for amphibolites yield extremely low zircon εHf(t) (−25.3 to −3.6) and whole-rock εNd(t) (−17.1 to −10.4) values respectively, corresponding to an enriched mantle source. However, in consideration of the fact that the recycled slab-derived fluids is difficult to alter the isotopic composition (especially hafnium) of lithospheric mantle compared with its geochemical characteristics. Meanwhile, on the compilation of all Precambrian subduction-related magmatism in the Yangtze Block, it appears that the enriched isotopic signature in amphibolites is mainly inherited from the Paleoproterozoic (ca. 2.0 Ga) lithospheric mantle beneath the Yangtze Block, and extremely negative εHf(t) values (–25.3 to –18.4) further reflect prominent crust-mantle interaction correspond to the assembly of Columbia supercontinents. Therefore, the Zhangbaling mafic rocks witness the modification of lithospheric mantle by two episodic subduction events during Paleoproterozoic and Neoproterozoic respectively, which provide new evidence for the Yangtze Block to get involved in the reconstruction of Columbia and Rodinia supercontinents.

Two contrasting Neoarchean metavolcanic rock suites in eastern Hebei and their geodynamic implications for the northern North China Craton

The late Archean was a crucial transition period for global geodynamics. Metavolcanic rocks, as one of the predominant components of Archean cratons, are key to deciphering the geodynamic regimes of early Earth crust-mantle interactions. Neoarchean supracrustal rocks in the central K-rich granitoid belt of the eastern Hebei-western Liaoning Province, northern North China Craton, may be subdivided into a lower metavolcanic sequence and an upper metavolcanic-sedimentary sequence by an angular unconformity.The lower sequence metavolcanic rocks consist primarily of 2558–2536 Ma amphibolite to granulite facies metamorphosed basaltic-andesitic rocks and high- and low-MgO dacitic rocks. The basaltic-andesitic samples display calc-alkaline affinity, high MgO contents, low (Nb/La)N and moderate (Hf/Sm)N values and were most likely derived from partial melting of mantle materials metasomatized by subducted slab- and sediment-derived melts and fluids. The high-MgO dacitic samples are characterized by strongly fractionated rare earth element (REE) patterns and relatively high MgO contents, which resemble typical high-SiO2 adakites. The magmatic precursors originated from subducted oceanic slabs and sediments. The samples with low-MgO dacite compositions have the highest SiO2 and lowest MgO contents, and the magmatic precursors were generated by partial melting of metamorphosed high-K mafic rocks and sedimentary rocks in the lower crustal level.The upper sequence metavolcanic rocks chiefly comprise greenschist to amphibolite facies metamorphosed tholeiites, tholeiitic to transitional basalts, calc-alkaline basalts, andesites, and dacites with magmatic crystallization ages of 2522–2511 Ma. The tholeiite samples are characterized by left-declined REE patterns and high (Nb/La)N and chondrite-like (Hf/Sm)N values resembling typical N-MORBs and were formed by partial melting of a depleted mantle source. The tholeiitic to transitional basalt samples resemble back-arc basin basalts and display weakly right-declined REE patterns, mildly negative Nb and Ti anomalies, and low (Nb/La)N and chondrite-like (Hf/Sm)N values. Their magmatic precursors were derived from partial melting of a slightly fluid- and melt-metasomatized depleted mantle source. The calc-alkaline basalt and andesite samples exhibit fractionated REE patterns and apparent Nb, Ta, and Ti depletions and originated from a strongly fluid- and melt-metasomatized depleted mantle source. The dacite samples show the highest SiO2 and lowest MgO contents in the upper sequence metavolcanic rocks and were derived from partial melting of lower crustal metasedimentary rocks. Systematic petrological, geochemical, and petrogenetic studies indicate that a mature continental arc setting and a back-arc basin setting may account for the formation of the lower and upper sequence metavolcanic rocks, respectively. Three stages of deformation can be identified in the K-rich granitoid belt, including 2525–2505 Ma extensional deformation in a back-arc basin setting (D1), ca. 2500 Ma top-to-the-NEE-SSE thrusting in a collisional orogenic setting (D2) and 2500–1800 Ma nearly N-S-oriented strike-slip shearing (D3), with D2 deformation accompanied by the synchronous occurrence of paired high-pressure and high-temperature medium-pressure granulite facies metamorphism. Integrating the petrogenesis, spatiotemporal relationships, deformational and metamorphic features of the Archean lithological assemblages, we consider that the late Neoarchean K-rich granitoid belt was formed in an active continental margin setting with a southeastward subduction polarity and experienced multi-stage trench retreats and a subsequent collision between the arc and continent.

Discovery of ∼409 Ma Amphibolite in the Zhikong‐Songduo Subduction Complex, Southern Tibet

Discovery of ∼409 Ma Amphibolite in the Zhikong‐Songduo Subduction Complex, Southern Tibet

A relic of the Mozambique Ocean in south-east Tanzania

The Neoproterozoic geology of East Africa records the events leading up to the formation of Gondwana and subsequently Pangea, and hence understanding the geodynamic history of this area offers a unique insight into supercontinent development. Eighteen new U-Pb zircon and two U-Pb monazite ages were collected in conjunction with 110 whole-rock lithogeochemistry analyses from a region of south-east Tanzania we designate the ‘Ntaka Terrane’. These new data suggest this area of the East African Orogen constitutes an exotic nappe terrane that was part of the Mozambique Ocean in the Neoproterozoic. The lithological sequence in the region consists of a 770–720 Ma ‘basement’ of intermediate-felsic paragneiss possibly derived from a volcano-sedimentary sequence and recording arc-like geochemical signatures. These are intruded by 700–650 Ma amphibolites (±garnet) with mafic-intermediate magmatic protoliths that also have arc-like geochemistry. This sequence is then intruded by a set of ca. 670–650 Ma ultramafic, dominantly pyroxenitic sills collectively termed the Ntaka Ultramafic Complex, which have a much more mantle-like, non-arc origin, possibly formed as the island-arc intersected a mantle plume ‘hot spot’. The mafic-intermediate intrusives occur synchronous with, and outlast, the high-Mg basalt magmatism, suggesting that this latter event was transient in the context of the overall tectonic regime. The final magmatic event in the terrane is the intrusion of 640–620 Ma syn-orogenic granites that are coeval with 640–600 Ma upper amphibolite-granulite facies metamorphism. The broader geodynamic setting consisted of the Congo-Tanzania-Bangweulu Block of east Africa to the west and the Azania Block to the east of the Neomozambique Ocean, both of which contain Archean-Paleoproterozoic detritus. Based on the island arc-like geochemical characteristics of the 700–650 Ma mafic intrusive rocks and the lack of detrital zircons >1000 Ma, we suggest that the Ntaka Terrane represents a Neoproterozoic intra-oceanic island arc which was accreted to either East Africa or Azania shortly before full continental collision (the East African Orogeny) at ca. 640–620 Ma. Data from the Ntaka Terrane correlate with the Vohibory Terrane (Madagascar), Eastern Granulites (northern Tanzania) and Cabo Delgado Nappe Complex (Mozambique), suggesting these areas represent a set of broadly correlative oceanic terranes.

Episodic slab rollback and back-arc extension in the Yunnan-Burma region: Insights from Cretaceous Nb-enriched and oceanic-island basalt–like mafic rocks

One highly debated topic related to the nature of the Cretaceous mantle and geodynamic processes in SE Asia is whether they were dominated by the Tethyan or Pacific Ocean subductions. The main reason for this puzzle is the lack of geochemical studies on mantle-derived mafic rocks in SE Asia. In this paper, we report Early Cretaceous Nb-enriched diabases (ca. 139 Ma) and Late Cretaceous Nb-enriched (ca. 68 Ma) and oceanic-island basalt (OIB)−like (ca. 73 Ma) amphibolites from the Ailaoshan tectonic zone in the Yunnan-Burma region. The ca. 139 Ma diabases and ca. 68 Ma amphibolites display similar geochemical features with typical Nb-enriched basalts, e.g., high Na 2 O, TiO 2 , and Nb (>7 ppm) contents, and high (Nb/Th) PM , (Nb/La) PM , and Nb/U ratios. Unlike adakite-associated Nb-enriched basalts, our Nb-enriched diabases and ampibolites were derived through mixing between OIB-like and arc-like mantle source components in the spinel stability field ( 2 and Nb (20.0−26.0 ppm) contents, and OIB-like rare earth element patterns and trace-element spidergrams. The OIB-like rocks were generated by a low degree of partial melting of garnet-facies peridotite (>80 km). In combination with previous Cretaceous to early Eocene age data from granitoids, three significant magmatic flare-ups are identified at 140−115 Ma, 75−64 Ma, and 55−50 Ma, with two magmatic gaps at 115−75 Ma and 64−55 Ma. Given the distances of the studied area from the Pacific and Neotethyan trenches (2400−5000 km and 320 km, respectively), these Cretaceous igneous rocks more likely formed in a Neotethyan subduction-related tectonic setting, rather than a Pacific Ocean−related setting. We propose a tectonic model involving episodic flat slab subduction, slab rollback, and intracontinental back-arc extension for Neotethyan subduction in the Yunnan-Burma region.

Eoarchean ophiolites? New evidence for the debate on the Isua supracrustal belt, southern West Greenland

The Eoarchean Isua supracrustal belt (Greenland) comprises two distinct packages that have been dated at ca. 3800 Ma and 3700 Ma. Both packages consist of strongly deformed, pillow lavas and lesser amounts of gabbro (island arc tholeiite, picrite and boninite protoliths), felsic schists (andesite-dacite protoliths), chemical sedimentary rocks and depleted mantle dunites, most are at amphibolite facies. This paper focuses on the ca. 3700 Ma assemblage in the northwestern arm of the belt. In a rare low strain area layered gabbro preserves an igneous texture and contains high Th/U igneous zircons with an age of 3720 to 3710 Ma. Boninitic amphibolites with relict pillow structure are cut by a 3712 ± 6 Ma tonalite sheet and an amphibolite-ultramafic schist tectonic contact is transgressed by a 3717 ± 6 Ma mafic tonalite intrusion. Strongly deformed felsic schists of likely volcanic origin contain 3720 to 3700 Ma igneous zircons, showing they are broadly contemporaneous with the intercalated mafic volcanic rocks. The 3720 to 3710 Ma supracrustal rocks are also intruded by 3696 ± 6 Ma tonalite. Hence Isua juvenile 3720 to 3710 Ma arc development was dominated by boninite, tholeiite and picrite eruption with mafic tonalite intrusion and evolved to maturity between 3710 to 3700 Ma, with formation of andesites, dacites, related volcano-sedimentary rocks and then intrusion of 3700 to 3690 Ma tonalites. The 3720 to 3710 Ma assemblage comprising mantle peridotite, gabbros, pillowed and non-pillowed volcanic rocks and chemical sedimentary rocks, is interpreted as an example of a ca. 3710 Ma ophiolite. However, it is not an intact section through an ophiolite for several reasons. First, this assemblage was repeatedly deformed and partitioned by Eoarchean shear zones and second, because a convincing “sheeted dike” complex has not yet been identified. Geochemical data from a candidate for sheeted dikes proposed by other workers does not match the geochemistry of nearby ca. 3700 Ma amphibolites with relict pillow structure, but instead better matches ca. 3510 Ma noritic Ameralik dykes. Nonetheless, the 3720 to 3690 Ma juvenile assemblage is juxtaposed to the south with a 3800 Ma complex, and hence it resembles Sierran-style ophiolites that are ensimatic island arc terranes accreted against older crust. This evidence supports the concept that crust-formation processes at convergent plate boundaries were operating almost 4 billion years ago.

Tectonic evolution of the southern Laurentian Grenville orogenic belt

Research Article| November 01, 1998 Tectonic evolution of the southern Laurentian Grenville orogenic belt Sharon Mosher Sharon Mosher 1Department of Geological Sciences, University of Texas at Austin, Austin, Texas 78712 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1998) 110 (11): 1357–1375. https://doi.org/10.1130/0016-7606(1998)110<1357:TEOTSL>2.3.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Sharon Mosher; Tectonic evolution of the southern Laurentian Grenville orogenic belt. GSA Bulletin 1998;; 110 (11): 1357–1375. doi: https://doi.org/10.1130/0016-7606(1998)110<1357:TEOTSL>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The Grenville orogenic belt along the southern margin of Laurentia records more than 300 m.y. of orogenic activity culminating in arc-continent and continent-continent collision ca. 1150–1120 Ma. Exposures in Texas provide a unique profile across the Grenville orogen from the orogen core to the cratonal margin. In the Llano uplift of central Texas, ca. 1360–1232 Ma upper amphibolite–lower granulite facies, polydeformed supracrustal and plutonic rocks represent the core of the collisional orogen. This exposure contains a suture between a 1326–1275 Ma exotic island-arc terrane and probable Laurentian crust and records A-type subduction. In west Texas, 1380–1327 Ma amphibolite to greenschist facies, polydeformed supracrustal rocks are thrust over ca. 1250 Ma carbonate and volcanic rocks along the cratonal margin. The carbonate and volcanic rocks form a narrow thrust belt with post–1123 Ma synorogenic sedimentary rocks, which grade into undeformed sedimentary rocks northward on the Laurentian craton.The Texas basement reveals a consistent but evolving tectonic setting for the southern margin of Laurentia during Mesoproterozoic time. This paper summarizes recent advances in our knowledge of the Texas basement and proposes plate models to explain the tectonic evolution of this margin during Mesoproterozoic time. The orogenic history is strikingly similar to that of the Canadian Grenville orogen and requires a colliding continent off the southern Laurentian margin during the assembly of Rodinia. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Palaeomagnetic study of the Nagssugtoqidian mobile belt in Central-West Greenland

This paper reports a palaeomagnetic study of amphibolite and granulite facies basement rocks across a 60 km wide north-south traverse through the Nagssugtoqidian mobile belt in West Greenland, which incorporates the c. 2500 Ma Itivdleq shear belt, the c. 1800 Ma Ikertôq shear belt, and adjoining terrains. It also integrates the results with earlier investigations of the belt to produce a composite record of the magnetic field during uplift and cooling of the terrain through c. 600–500°C at c. 1750–1600 Ma. Progressive a.f. and thermal cleaning of samples from 81 sites identifies a range of magnetic components. Lowest blocking temperature remanences in Nagssugtoqidian (c. 1700 Ma) amphibolite facies rocks yield steep W to N positive components residing in magnetite. Granulite facies rocks formed at c. 1800 Ma generally have a dual record incorporating steep N to NW positive components unblocked by 580°C and shallow E-W directed components of dual polarity only unblocked close to the Curie point of haematite. A common feature of relict c. 2500 Ma amphibolite and granulite facies terrains is a progressive movement of sample directions back along a palaeofield migration path with treatment and is interpreted to represent the recovery of a discrete spectrum acquired during a.p.w. Areas with a distributed spectra through the critical range of blocking temperatures, altitude-controlled sections, and blocking temperature relationships all record consistent a.p.w. during the interval of uplift-related cooling. The collective data define a clockwise palaeofield migration path between D = 174, I = 32°, and D = 348, I = 66° referred to as the ‘A’ magnetisation. The equivalent palaeopoles define one limb of an a.p.w. loop conforming to other Laurentian poles assigned to the interval 1750–1650 Ma and widely represented as a post-Hudsonian overprinting or cooling remanence. Shallow E-W directions, designated ‘B’, predominate in late tectonic diorites and are therefore younger than 1700 Ma; they appear to represent a localised reheating event near the northern margin of the Ikertôq shear belt, possibly related to this magmatism, which produced the hemoilmenite phases. The equivalent palaeopoles are provisionally interpreted to represent part of the return path of the APW loop because they are coincident with other poles dated c. 1630 Ma. Collectively the palaeofield migration data from all studies define a thermal dome broadly coincident with the Ikertôq shear belt, but it is not possible to isolate the contribution of components of vertical movement along faults running through the belt to this regional effect.