scholarly journals Mafic microgranular enclaves formed by gas-driven filter pressing during rapid cooling: An example from the Gangdese Batholith in southern Tibet

2021 ◽  
Author(s):  
Wei Xu ◽  
Di-Cheng Zhu ◽  
Qing Wang ◽  
Roberto F Weinberg ◽  
Rui Wang ◽  
...  
Keyword(s):  
Magma Ascent ◽  
Southern Tibet ◽  
Significant Information ◽  
Rapid Cooling ◽  
Igneous Complex ◽  
Fine Grained ◽  
Arc Magmas ◽  
Microgranular Enclaves ◽  
Mafic Microgranular Enclaves ◽  
Compatible Elements

Abstract Mafic microgranular enclaves (MMEs), widespread in intermediate to felsic arc plutons, carry significant information on the genesis and evolution of arc magmas, yet their origin remains debatable. Here, we examine MME-host diorite pairs from the ca. 200 Ma Cuijiu Igneous Complex in the eastern Gangdese Batholith, southern Tibet, to constrain the petrogenesis of MMEs and the evolution of arc magmas. Within the complex, MMEs are essentially similar to their host diorites with similar emplacement ages (∼200 Ma), mineral assemblages and mineral compositions, as well as whole-rock Sr–Nd–Hf and zircon Hf isotopic compositions. However, MMEs have higher modal contents of hornblende and biotite, and are enriched in compatible elements and depleted in incompatible elements. Zircons from some MME samples are characterized by dark-CL cores overgrown by light-CL rims of varying thickness. The dark-CL cores show higher Th, U, and rare earth elements (REE) abundances than the light-CL rims. Based on comparison with co-genetic mafic melts and mass-balance calculations, we propose that the MMEs were early-crystallized cumulates (autoliths) related to their host diorites. The chilled textures, flow microstructures and pillow shapes suggest that the MMEs experienced rapid cooling before being captured by the host magmas. The rapid cooling may result from contact between ascending diorite magmas and cooler wall rocks. As the magmas quickly crystallized, they reached second boiling and vesiculation, and separated into fine-grained crystal-rich margins and melt-rich centers. Gradients in crystallinity and pressure expelled interstitial melts from the crystal-rich margins to the crystal-poor centers, leading to crystal-liquid separation (gas-driven filter pressing). The dark-CL zircon cores with high Th and U abundances may crystallize from highly evolved interstitial melts within the crystal-rich margins. The fine-grained crystal-rich margins were subsequently captured and dragged as MMEs before their complete crystallization by later ascending host magmas. This differentiation process could have occurred over several kilometers of magma ascent, and have played an important role in the polybaric fractional crystallization of the Cuijiu Igneous Complex, feeding more differentiated andesitic magmas to upper crustal mushes.

Cumulate mush hybridization by melt invasion: Evidence from compositionally-diverse amphiboles in ultramafic-mafic arc cumulates within the eastern Gangdese Batholith, southern Tibet

2021 ◽  
Author(s):  
Wei Xu ◽  
Di-Cheng Zhu ◽  
Qing Wang ◽  
Roberto F Weinberg ◽  
Rui Wang ◽  
...  
Keyword(s):  
Mineral Assemblage ◽  
Large Body ◽  
The Body ◽  
Coarse Grained ◽  
Southern Tibet ◽  
Fine Grained ◽  
Arc Magmas ◽  
Rock Types ◽  
Geochemical Study ◽  
Host Rocks

Abstract Amphibole plays an important role in the petrogenesis and evolution of arc magmas, but its role is not completely understood yet. Here, a field, petrological, geochronological and geochemical study is carried out on ultramafic-mafic arc cumulates with textural and chemical heterogeneities and on associated host diorites from the eastern Gangdese Batholith, southern Tibet to explore the problem. The cumulates occur as a large body in diorite host-rocks. The core of the body consists of coarse-grained Cpx hornblendite with a porphyritic texture. Towards the contact with the host diorite, the coarse-grained Cpx hornblendite grades to relatively homogeneous fine-grained melagabbro. Zircon U–Pb dating indicates they all crystallized at 200 ± 1 Ma. Textural features and whole-rock and mineral chemical data reveal that both the Cpx hornblendite and the melagabbro are mixtures of two different mineral assemblages that are not in equilibrium: (1) brown amphibole and its clinopyroxene inclusions; (2) matrix clinopyroxene + green amphibole + plagioclase + quartz + accessory phases. Clinopyroxene and brown amphibole from the first assemblage are enriched in middle rare earth elements (MREE) relative to light REE (LREE) and heavy REE (HREE), and are weakly depleted in Ti, whereas clinopyroxene and green amphibole from the second assemblage are characterized by LREE enrichment over MREE-HREE and more marked Sr and Ti depletion. The higher Mg#, MgO and Cr of the late-formed green amphibole than the early-formed brown amphibole suggest that the two assemblages are not on the same liquid line of descent. Given the close relations of the three rock types in the exposed crustal section, the cumulates are interpreted to have formed in an open system, in which an ultramafic cumulate body consisting of the first assemblage reacted with the host dioritic melt to form new clinopyroxene and amphibole of the second assemblage. The melt calculated to be in equilibrium with the first mineral assemblage resembles an average continental arc basalt, that is less evolved than the host dioritic melt, responsible for the second mineral assemblage. On the basis of whole-rock Sr–Nd–Hf isotopic similarity of the cumulates and a host diorite sample, we argue that the host diorites were formed through crystal fractionation from the parent melt of the first assemblage. Results of least-squares mass-balance calculations suggest the quantities of the host dioritic melts, involved in the generation of these modified cumulates, vary from ~25% to ~44%. The presence of magmatic epidote in the host diorites and Al-in-Hb geobarometry indicate the reaction that occurred when the dioritic melts percolated through the cumulate body was at ~6 kbar. Both the brown and green amphiboles are enriched in MREE relative to HREE, and can impart residual melts with a strong geochemical signature of amphibole fractionation (low Dy/Yb). Thus, we conclude that fractional crystallization and melt-rock reaction are two mechanisms by which amphibole controls arc magma petrogenesis and evolution.

Ferromagnesian silicate association in S-type granites: the Darongshan granitic complex (Guangxi, South China)

2008 ◽  
Vol 179 (1) ◽  
pp. 13-27 ◽  
Author(s):  
Bernard Charoy ◽  
Pierre Barbey
Keyword(s):  
South China ◽  
Granulite Facies ◽  
Coarse Grained ◽  
Magma Ascent ◽  
Granulite Facies Metamorphism ◽  
Rock Types ◽  
Microgranular Enclaves ◽  
Mafic Microgranular Enclaves ◽  
Type Granite ◽  
Granite Complex

Abstract The late Indosinian Darongshan granite complex (Guangxi Province, South China) consists mainly of three plutons (Taima, Jiuzhou and Darongshan), with coarse-grained to subvolcanic rock types. There is a rough mineral evolution from the western to the eastern part of the complex, with ferromagnesian magmatic silicates sequentially distributed : Opx+Crd±Bt (Taima), Opx+Grt+Bt+Crd (Jiuzhou) and Bt+Crd (Darongshan). Restitic, cumulative or xenocrystic minerals (mainly Crd with fibrolite+spinel inclusions, Grt and probably Opx in some cases) are also encountered. Mineralogical, chemical and isotopic compositions of the granites suggest that the three plutons derive from a source dominated by a reduced, immature greywacke-psammite series, which has experienced high-amphibolite to granulite-facies metamorphism. This S-type granite complex is considered to result from various degrees of melting at different crustal levels by biotite dehydration-melting in a high T (800° to 950°C), low P (400–600 MPa) metamorphic environment. Abundant granulite-facies metasedimentary enclaves with refractory compositions are considered as xenoliths entrapped during magma ascent. The presence of scarce mafic microgranular enclaves and the high temperatures needed for melting could suggest that heat may have been partly contributed by mantle-derived magmas.

The role of shallow open system processes in the evolution of South Tepeldag Pluton (NW Turkey): insights and constraints from thermodynamic modelling

2021 ◽  
Author(s):  
Tunahan Arık ◽  
Ömer Kamacı ◽  
Işıl Nur Güraslan ◽  
Şafak Altunkaynak
Keyword(s):  
Fractional Crystallization ◽  
Open System ◽  
Suture Zone ◽  
Thermodynamic Simulations ◽  
Nw Turkey ◽  
Microgranular Enclaves ◽  
Mafic Microgranular Enclaves ◽  
Ophiolitic Rocks ◽  
Compositional Variations ◽  
Spider Diagrams

<p>Eocene granitoids in NW Anatolia occurred following the continental collision between Sakarya Continent and Tauride-Anatolide Platform and mark the onset of post-collisional magmatism in the region. One of the representative members of the Eocene granitoids, the Tepeldağ pluton crops out as two isolated granitic bodies and is intruded into the Cretaceous blueschist assemblages (Kocasu formation) and ophiolitic rocks within the Izmir-Ankara-Erzincan suture zone (IAESZ). South Tepeldağ pluton (STP) is composed mainly of granodiorite with subordinate quartz diorite, which show transitional contacts. Aplitic dykes crosscut the pluton as well as the country rocks. STP includes a number of mafic microgranular enclaves (MME) of gabbro/diorite composition.</p><p>Geochemically, STP shows distinct I-type affinity with a metaluminous to slightly peraluminous (ASI ≤1.02) nature. The samples are medium-K to high-K calc-alkaline in character. They exhibit depletion in HFSE (Ti, Hf, Zr, Nb and Ta) compared to large ion lithophile elements (Rb, Ba, Th, U, K) and presents negative Nb, P, Ti anomalies. STP displays slight negative Eu anomalies (Eu/Eu* = 0.7–1.2), enrichment in LREE and flat HREE patterns in chondrite-normalized spider diagrams. MELTS modeling (with initial parameters of 1–3 kbar pressure, 2–3% water and QFM-NNO oxygen fugacity buffers) indicate that compositional variations in STP samples can be interpreted as a result of open system processes (assimilation fractional crystallization) rather than a reflection of fractional crystallization in the upper crustal magma chamber. All thermodynamic simulations dictate a crustal assimilation, especially in the late stages of the magmatic process, with a MgO, Na<sub>2</sub>O and Al<sub>2</sub>O<sub>3</sub>-rich assimilant similar to the suture zone (IAESZ) rocks.</p>

Sign in / Sign up

Export Citation Format

Share Document