Geochemical and Nd, Sr Isotopic Study of the Post‐Orogenic Granites in the Yidun Arc Belt of Northern Sanjiang Region, Southwestern China
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Abstract: In the arc (basin)–back area of the Yidun arc belt in the north segment of the Sanjiang tectonic zone, southwestern China, there occurs a post‐orogenic granite belt extending for more than 300 km in NNW direction. It strides across two different tectonic units of the arc (basin)–back area and the subduction area, and is accompanied by extensive Ag‐Sn polymetal–lic mineralizations. More than ten granite bodies have very similar geochemical characteristics: high SiO 2 (73.8–76.3 wt%) and K 2 O+Na 2 O (7.16‐8.41 %), and low Al 2 O 3 (11.9–13.6 %), CaO (0.46‐1.54 %) and MgO (0.16‐0.61 %), as well as high enrichment of Nb, Ta, Ga and Y, and strong depletion of Sr and Eu. Most of these features are peculiar to A‐type granite. Rb‐Sr and 40 Ar/ 39 Ar isotopic dating results indicate that the formation ages of the granites decrease from 103.7 Ma of the north end to 75.2 Ma near the south end, and that the magmatism became younger from north to south. The tectonic environment analysis clearly reveals that they were formed in post‐orogenic within–plate extension settings. The magma genesis was controlled by a united crustal extension regime after the arc‐continent collision. The granites have low Nd values ranging from –4.96 to –8.40, whereas the Sr values vary greatly ranging from –31.7 to 296, reflecting that the source composition transited from mantle – differentiated igneous rocks in the north to basement – dominated metamorphosed sedimentary rocks in the south. Under high temperature and water‐absent conditions, the anatexes of the crustal rocks made a great amount of plagioclase separated from melts and left in magma sources. Through this mechanism, the post‐orogenic granites took geo‐chemical characteristics such as low Al 2 O 3 and CaO, and strong depletion of Sr and Eu.Keywords:
Back-arc basin
Island arc
Abstract Extension within a continental back‐arc basin initiates within continental rather than oceanic lithosphere, and the geochemical characteristics of magmatic rocks within continental back‐arcs are poorly understood relative to their intraoceanic counterparts. Here, we compile published geochemical data from five exemplar modern continental back‐arc basins—the Okinawa Trough, Bransfield Strait, Tyrrhenian Sea, Patagonia plateau, and Aegean Sea/Western Anatolia—to establish a geochemical framework for continental back‐arc magmatism. This analysis shows that continental back‐arcs yield geochemical signatures more similar to arc magmatism than intraoceanic back‐arcs do. We apply this framework to published data for Triassic‐Jurassic magmatic rocks from the Caucasus arc system, which includes a relict continental back‐arc, the Caucasus Basin, that opened during the Jurassic and for which the causal mechanism of formation remains debated. Our analysis of 40 Ar/ 39 Ar and U‐Pb ages indicates Permian‐Triassic arc magmatism from ∼260 to 220 Ma due to subduction beneath the Greater Caucasus and Scythian Platform. Late Triassic (∼220–210 Ma) collision of the Iranian block with Laurasia likely induced trench retreat in the Caucasus region and led to migration of the Caucasus arc and opening of the Caucasus Basin. This activity was followed by Jurassic arc magmatism in the Lesser Caucasus from ∼180 to 140 Ma and back‐arc spreading in the Caucasus Basin from ∼180 to 160 Ma. Trace element and Sr‐Nd isotopic data for magmatic rocks indicate that Caucasus Basin magmatism is comparable to modern continental back‐arcs and that the source to the Lesser Caucasus arc became more enriched at ∼160 Ma, likely from the cessation of back‐arc spreading.
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The age and style of opening of the Havre Trough back-arc system is uncertain due to a lack of geochronologic constraints for the region. 40Ar/39Ar dating of 19 volcanic rocks from across the southern Havre Trough and Kermadec Arc was conducted in three laboratories to provide age constraints on the system. The results are integrated and interpreted as suggesting that this subduction system is young (<2 Ma) and coeval with opening of the continental Taupo Volcanic Zone of New Zealand. Arc magmatism was broadly concurrent across the breadth of the Havre Trough.
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The research significance and history,situations and problems concerned as well as development and prospecting of further deep study on volcanic rocks from Okinawa Trough,have been briefly expounded in this paper.It is emphasized that,for the different kinds of volcanic rocks in Okinawa Trough,typical active back arc basin in the initial stage of spreading,the study should be based on the modern theory and technique of geochemistry and also on the integrated induction and thinking in view of magmatism in arc trench system.Futhermore,our study should also pay attention to either the genetic link among the different series of rocks,or the link between magmatism and tectonic environment.The characters,regularity and sign of magmatism during the initial spreading of back arc basin should be revealed through comparing with the volcanic rocks from mature back arc basin,so as to make clear dynamic process during the initial spreading stage of the back arc basin.
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Metasomatism
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Eclogitization
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Available data enable the recognition of three periods of back-arc crustal generation and three pulses of volcanic activity along the associated island arcs of the Philippine Sea. The geochemistry of the basalts from the back-arc basins of different ages indicates that in most cases they are identical to mid-ocean ridge basalts, and therefore should have similar sources and origins. In contrast, the island arc rock composition is variable in space and time, reflecting the different source and more complex nature of corresponding magmatism. Geomagnetic studies and recent Deep Sea Drilling Project results suggest an alternate sequence of back-arc and arc magmatic cycles. Both geochemical and geological observations provide important constraints on models of magmatism and extensions tectonics at convergent plate boundaries.
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Abstract Back‐arc basins are essential geological units, formation and development of which are controlled by subduction dynamics. In the previous modeling studies, evolution of back‐arc basins was widely investigated under single subduction scenarios, suggesting that development of a back‐arc basin is only influenced by the adjacent single subduction zone. However, natural observations show that in the case of face‐to‐face double subduction, such as the New Hebrides and Tonga subduction systems in the Southwest (SW) Pacific region, evolution of back‐arc basins is likely affected by double subduction. How double subduction affects back‐arc basin evolution remains enigmatic. Here, we use 2D thermomechanical numerical modeling to investigate the dynamic evolution of back‐arc basins affected by double subduction. Our results suggest that episodic back‐arc spreading could be induced by double subduction, i.e., back‐arc spreading is first retarded by the development of a second subduction and then promoted by its slab break‐off. The double subduction further enhances upwelling of the deep mantle, facilitating the material circulation between the shallow and deep mantle. The major parameters influencing double subduction and back‐arc spreading are the distance between the two subduction zones and slab ages. Our results provide a new perspective for interpreting the episodic back‐arc spreading and the enriched back‐arc basalts in the SW Pacific region, and suggest the importance of double subduction in the geodynamic evolution of this region.
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We investigate the temporal record of magmatism in the Fiordland sector of the Median Batholith (New Zealand) with the goal of evaluating models for cyclic and episodic patterns of magmatism and deformation in continental arcs. We compare 20 U-Pb zircon ages from >2300 km2 of Mesozoic lower and middle crust of the Western Fiordland Orthogneiss to existing data from the Median Batholith to: (1) document the tempo of arc construction, (2) estimate rates of magmatic addition at various depths during arc construction, and (3) evaluate the role of cyclical feedbacks between magmatism and deformation during high and low magma addition rate events. Results from the Western Fiordland Orthogneiss indicate that the oldest dates are distributed in northern and southern extremities: the Worsley Pluton (123–121 Ma), eastern McKerr Intrusives (128–120 Ma), and Breaksea Orthogneiss (123 Ma). Dates within the interior of the Western Fiordland Orthogneiss (Misty and Malaspina Plutons, western McKerr Intrusives) primarily range from 118 to 115 Ma and signify a major flux of mafic to intermediate magmatism during which nearly 70% of the arc root was emplaced during a brief, ∼3 m.y., interval. The spatial distribution of dates reveals an inward-focusing, arc-parallel younging of magmatism within the Western Fiordland Orthogneiss during peak magmatic activity. Coupled with existing data from the wider Median Batholith, our data show that Mesozoic construction of the Median Batholith involved at least two high-flux magmatic events: a surge of low-Sr/Y plutonism in the Darran Suite from ca. 147 to 136 Ma, and a terminal surge of high-Sr/Y magmatism in the Separation Point Suite from 128 to 114 Ma, shortly before extensional collapse of the Zealandia Cordillera at 108–106 Ma. Separation Point Suite magmatism occurred at all structural levels, but was concentrated in the lower crust, where nearly 50% of the crust consists of Cretaceous arc-related plutonic rocks. Existing isotopic data suggest that the flare-up of high-Sr/Y magmatism was primarily sourced from the underlying mantle, indicating an externally triggered, dynamic mantle process for triggering the Zealandia high–magma addition rate event, with only limited contributions from upper plate materials.
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