Elsevier

Lithos

Volumes 162–163, March 2013, Pages 157-174
Lithos

Zirconological tracing of transition between aqueous fluid and hydrous melt in the crust: Constraints from pegmatite vein and host gneiss in the Sulu orogen

https://doi.org/10.1016/j.lithos.2013.01.004Get rights and content

Abstract

Zircon can grow from aqueous fluid and hydrous melt in crustal rocks when they undergo metamorphic dehydration and partial melting under high-pressure (HP) to ultrahigh-pressure (UHP) conditions. A distinction between fluid and melt has important bearing on the development of crustal anatexis in collisional orogens. A genetic transition between fluid and melt is recorded by zircons from pegmatite vein and host UHP gneiss in the Sulu orogen. This transition is elucidated by an integrated study of SIMS U–Pb and O isotope analyses with LA-ICPMS U–Pb isotope and trace element analyses in the zircons. Pegmatite veins yield zircon U–Pb ages of 147–153 Ma for new growths, and 700–800 Ma for relict cores. A one meter-width pegmatite vein exhibits two episodes of zircon growth at 153 ± 3 Ma for inner domain and at 147 ± 2 Ma for outer domain. The two types of domains have a series of differences in CL image, inclusion type, REE content and pattern, trace element contents and ratios, and Ti-in-zircon temperature. The inner domain grew from the hydrous melt at 730–840 °C, whereas the outer domain grew from the aqueous fluid at 520–650 °C. Nevertheless, they have similarly low δ18O values of 1.0–2.3‰, suggesting their growth from O isotope homogeneous media despite the transition from melt to fluid. The host gneiss of this pegmatite vein exhibits three generations of zircon growth, with 180–205 Ma at 700–770 °C during amphibolite-facies metamorphism, 157 ± 3 Ma at 610–670 °C for fluid-assisted growth, and 147 ± 2 Ma at 780–860 °C for melt-assisted growth. The last two types of growth record the development of crustal anatexis from hydration melting to dehydration melting. The other decimeter-width pegmatite vein exhibits only one episode of zircon growth from melt at 149 ± 2 Ma and 660–860 °C. Its host gneiss exhibits a residual melt texture but no significant growth of zircon, indicating low degree of dehydration melting. Therefore, the zircon domains in the host gneisses record their growth during a transition from aqueous fluid to hydrous melt along a temperature-increasing path, whereas the zircon domains in the pegmatite veins record their growth during a transition from hydrous melt to aqueous fluid along a temperature-decreasing path. Despite the difference in the direction of fluid/melt evolution, the all zircon domains in these gneisses and pegmatite veins record the same event of crustal anatexis in the UHP gneisses. The present study also provides insights into a genetic definition of anatectic melt and thus anatectic zircon.

Highlights

► Zircon domains grown from aqueous fluid and hydrous melt are identified in pegmatite vein and host gneiss. ► The progress of dehydration melting is indicated by a genetic transition from fluid to melt in the gneiss. ► The effect of mineral crystallization is indicated by a genetic transition from melt to fluid in the pegmatite vein. ► Breakdown of hydrous minerals in the gneiss is responsible for fluid focus and crustal anatexis. ► Anatectic zircon is distinguished from magmatic zircon by a series differences in trace element composition.

Introduction

Aqueous fluid and hydrous melt are two principal species of geological fluid under high-pressure (HP) to ultrahigh-pressure (UHP) conditions (e.g., Hermann et al., 2006, Zheng et al., 2011). They are distinguished from each other by differences in physicochemical property and geochemical composition. They occur in high-grade metamorphic terranes as metamorphic fluid and melt as well as magmatic fluid and melt, respectively (e.g., Zheng, 2012). The metamorphic fluid and melt are produced by dehydration and anatexis, respectively, during high-grade metamorphism under amphibolite-, eclogite- and granulite-facies conditions. The metamorphic melt is also referred to as anatectic melt that has not escaped from parental metamorphosed rocks such as migmatite and granulite. In contrast, the magmatic melt has escaped from its source rocks through significant ascent and accumulation of anatectic melt. The magmatic fluid has evolved and separated from the magmatic melt because the solubility of water in hydrous melt decreases with decreasing pressure–temperature (P-T) during magma emplacement. Although the genetic classification between the metamorphic and magmatic fluids/melts is straightforward, it is challenging to distinguish them by means of mineralogical records. Because the difference in the extent of incompatible element saturation is a critical parameter, minerals crystallized from the fluids/melts are important targets to investigate.

Aqueous fluid plays a critical role in anatexis of the continental crust under HP to UHP conditions (e.g., Zheng et al., 2011). Within the crust, anatexis may be caused by an ingress of aqueous fluid into crustal rocks via fluid-induced (hydration) melting (e.g., Berger et al., 2008, Prince et al., 2001, Rubatto et al., 2009), or by breakdown of hydrous minerals as dehydration-driven melting (e.g., Brown, 2010, Skjerlie and Patiño Douce, 2002, Zheng et al., 2011). The former takes place at the wet solidus with the saturation of water, whereas the latter occurs at elevated temperatures above the wet solidus with the undersaturation of water. In either case, silicate melts are produced with variable amounts of water, depending on the contents of water in the parental rocks and the P-T conditions and timescale of partial melting. This has important bearing on the solution of incompatible elements in hydrous melts because water behaves like the most incompatible element during partial melting and fractional crystallization. There is also genetic relationship between source and sink for water and incompatible elements in crustal rocks (Zheng, 2009, Zheng, 2012).

It has been noted petrologically that the amount of hydrous melts derived from partial melting of granitic rocks sometimes far exceeds what can be produced through the breakdown of hydrous minerals in the rocks (e.g., Berger et al., 2008, Burri et al., 2005, Slagstad et al., 2005). Petrogenetic modeling also demonstrated that only a small percentage of melt can be generated from the breakdown of micas at temperatures in excess of 800 °C in a granitic rock (Schulmann et al., 2008). This suggests involvement of additional water from somewhere, either external or internal origins. The internal fluid can be generated by the exsolution of structural hydroxyl and molecular water in nominally anhydrous minerals (Zheng, 2009). Furthermore, partial melting of granitic rocks may initially take place at the wet solidus primarily due to local accumulation of aqueous fluids (Sawyer, 2010, Zheng et al., 2011). In this regard, there exists the evolution from metamorphic aqueous fluid to anatectic hydrous melt in the lithological system with increasing temperature. The segregation, ascent and accumulation of anatectic melt, on the other hand, can eventually evolve to magmatic melt, in which magma results after crystallization of significant amounts of rock-forming minerals. During magma evolution, aqueous fluid would approach saturation in magmatic melt and eventually exsolved as magmatic fluid. Thus, there exists the evolution from hydrous melt to aqueous fluid in the magmatic system. In this context, metamorphic fluid, anatectic melt, magmatic melt and magmatic fluid may be generated by continuous processes with changing P-T conditions.

Granitic pegmatite is an example that crystallizes from hydrous melt to aqueous fluid (Simmons and Webber, 2008, Thomas et al., 2000). Some pegmatite veins were formed from partial melting of granitic rocks (London, 2005) and also reported in UHP metagranite in the Sulu orogen of China (e.g., Liu et al., 2010a, Wallis et al., 2005). These pegmatite veins, together with their parental rocks, are excellent samples to study the generation and evolution of aqueous fluid and hydrous melt during regional metamorphism and crustal anatexis. However, it is difficult in petrology to trace the transition between aqueous fluid and hydrous melt because the evidence of hydration melting was mostly erased by subsequent dehydration melting at elevated temperatures (Rubatto et al., 2009).

Zircon, a common accessory mineral in high-grade metamorphic rocks, can readily crystallize from aqueous fluid (e.g., Chen et al., 2011a, Chen et al., 2012, Rubatto and Hermann, 2003, Wu et al., 2009, Zheng et al., 2007) and hydrous melt (e.g., Liu et al., 2010a, Rubatto, 2002, Rubatto et al., 2009, Vavra et al., 1996, Xia et al., 2009, Zong et al., 2010). Due to its robustness, refractory property and extremely low diffusion rates for many elements, zircon can commonly retain its growth age, trace element and isotope signatures even if exposed to suprasolidus temperatures (Cherniak and Watson, 2003, Scherer et al., 2007, Zheng et al., 2004). Thus, zircon U–Pb dating is widely used to determine the time of its growth from aqueous fluid and hydrous melt. Furthermore, the trace element composition of zircon is used to distinguish magmatic origin from metamorphic and hydrothermal origins (e.g., Chen et al., 2010, Chen et al., 2012, Hinton and Upton, 1991, Hoskin, 2005, Rubatto, 2002, Whitehouse and Platt, 2003, Wu et al., 2009, Xia et al., 2009, Xia et al., 2010, Zheng, 2009). However, such distinction by the REE composition alone has encountered difficulties in distinguishing hydrothermal zircon from magmatic and metamorphic zircons (e.g., Fu et al., 2009, Hoskin and Schaltegger, 2003, Pettke et al., 2005). So does in distinguishing anatectic zircon from magmatic and metamorphic zircons.

Substantially, the magmatic melt can be viewed as a highly evolved product of the anatectic melt. While it may be originally produced by high-degree partial melting rather than low-degree anatexis in the source, it has also experienced protracted segregation, homogenization and fractional crystallization. As a consequence, the magmatic melt is commonly assumed to have achieved thermodynamic equilibrium in petrology and geochemistry. While the most evolved anatectic melt cannot be distinguished from the magmatic melt, the least evolved anatectic melt cannot be distinguished from the metamorphic fluid. However, a reasonable distinction between the two origins of hydrous melt is very important in petrogenetic and geochemical studies. Zirconology, the integrated study of zircon mineragraphy (internal structure and external morphology), U–Pb geochronology, mineral inclusions, trace elements, Ti content thermometer, and Lu-Hf and O isotope compositions, can potentially distinguish anatectic melt from magmatic melt and metamorphic fluid. In this paper, we present a zirconological study of granitic pegmatite veins and host rock UHP granitic gneisses in the Sulu orogen. The results not only provide geochemical distinction between zircon growths from aqueous fluid and hydrous melt, but also place constraints on the processes of crustal anatexis with regard to the transition between aqueous fluid and hydrous melt.

Section snippets

Geological setting and samples

The Dabie–Sulu orogenic belt in east-central China was formed by subduction of the South China Block beneath the North China Block in the Triassic (e.g., Cong, 1996, Zhang et al., 2009, Zheng et al., 2005). The Sulu orogen is considered as its eastern part, with an offset of about 500 km to the northeast along the Tan–Lu fault. This orogen is bounded by the Jiashan–Xiangshui fault in the south and the Wulian–Qingdao–Yantai fault in the north (Fig. 1). It consists of a fault-bounded HP

Analytical methods

Zircon grains were collected from samples and purified by hand picking under a binocular microscope. Representative zircon grains were mounted in epoxy, and then polished down to expose the grain centers. All zircon grains were documented under transmitted and reflected light as well as cathodoluminescence (CL) images to reveal their external morphology and internal structure as a guide for in-situ microbeam analyses.

CL images were collected by a FEI Sirion200 Scanning electron microscope at

Results

Three samples (PV1, HG2 and PV2) were analyzed by SIMS for zircon U–Th–Pb and O isotopes, and all four samples were analyzed by LA-ICPMS for zircon U–Th–Pb isotopes and trace elements. All data of the SIMS and LA-ICPMS analyses are listed in Appendices Tables A1 to A4. Table 1 presents a summary of zircon U–Pb ages, O isotopes and trace elements for the samples. Calculation of Ti-in-zircon temperatures is based on the calibrations of Ferry and Watson (2007). Because all the samples contain

Discussion

In order to correlate SIMS zircon O and U–Th–Pb data with LA-ICPMS U–Th–Pb isotope and trace element data, CL images and microscopic observations were used as a guide to ensure that all the in-situ analyses were operated at the same position for each spot (Chen et al., 2011a, Sheng et al., 2012). For SIMS U–Th–Pb and O isotope analyses, although the sample domain is not strictly the same because about 5 μm thickness layer was polished after the SIMS U–Th–Pb isotope analysis, they were performed

Transition between aqueous fluid and hydrous melt

If a free aqueous fluid is present in crustal rocks, partial melting starts on the wet solidus, commonly at 650–700 °C, generating water-saturated melt (Brown, 2010, Zheng, 2012). After the hydration melting used up the available free water and in the absence of external water, dehydration melting takes place to produce water-undersaturated melt at temperatures above the wet solidus, commonly at 750–800 °C (Brown, 2010, Clemens, 2006). In either case, melts generated are hydrous with the

Conclusions

Microbeam in-situ zircon U–Pb dating indicates multiple episodes of zircon growth in pegmatite veins and their host UHP gneisses in the Sulu orogen. A meter-wide pegmatite vein and its host gneiss yield two episodes of zircon growth at 153 ± 3 and 147 ± 2 Ma, and 157 ± 3 and 147 ± 2 Ma, respectively. The host gneiss also records zircon growth during amphibolite-facies metamorphism at 180–205 Ma. A centimeter-width pegmatite vein only yields one episode of zircon growth at 149 ± 2 Ma; its host gneiss does not

Acknowledgments

This study was supported by funds from the National Ministry of Science and Technology (2009CB825004), the Natural Science Foundation of China (41221062, 41173013 and 40921002), the Fundamental Research Programs for the Central Universities (WK2080000018 and WK2080000032) and the Program for New Century Excellent Talents in University (NCET-11-0875). Thanks are due to Yu Liu, Guoqiang Tang and Xianhua Li for their assistance with the SIMS zircon U–Pb and O isotope analyses, to Yongsheng Liu and

References (122)

  • R.-X. Chen et al.

    Episodic fluid action during exhumation of deeply subducted continental crust: geochemical constraints from zoisite–quartz vein and host metabasite in the Dabie orogen

    Lithos

    (2012)
  • Y.-X. Chen et al.

    Synexhumation anatexis of ultrahigh-pressure metamorphic rocks: petrological evidence from granitic gneiss in the Sulu orogen

    Lithos

    (2013)
  • E. Dubińska et al.

    U–Pb dating of serpentinization: hydrothermal zircon from a metasomatic rodingite shell (Sudetic ophiolite, SW Poland)

    Chemical Geology

    (2004)
  • B. Fu et al.

    Distinguishing magmatic zircon from hydrothermal zircon: a case study from the Gidginbung high-sulphidation Au–Ag–(Cu) deposit, SE Australia

    Chemical Geology

    (2009)
  • J. Hermann

    Allanite: thorium and light rare earth element carrier in subducted crust

    Chemical Geology

    (2002)
  • J. Hermann et al.

    Aqueous fluids and hydrous melts in high-pressure and ultra-high pressure rocks: implications for element transfer in subduction zones

    Lithos

    (2006)
  • R.W. Hinton et al.

    The chemistry of zircon-variations within and between large crystals from syenite and alkali basalt xenoliths

    Geochimica et Cosmochimica Acta

    (1991)
  • P.W.O. Hoskin

    Trace-element composition of hydrothermal zircon and the alteration of Hadean zircon from the Jack Hills, Australia

    Geochimica et Cosmochimica Acta

    (2005)
  • S.E. Jackson et al.

    The application of laser ablation-inductively coupled plasma-mass spectrometry to in situ U–Pb zircon geochronology

    Chemical Geology

    (2004)
  • X.H. Li et al.

    Petrogenesis and tectonic significance of the 850 Ma Gangbian alkaline complex in South China: evidence from in situ zircon U–Pb dating, Hf–O isotopes and whole-rock geochemistry

    Lithos

    (2010)
  • F.L. Liu et al.

    Zircon as the best mineral for P-T time history of UHP metamorphism: a review on mineral inclusions and U–Pb SHRIMP ages of zircons from the Dabie–Sulu UHP rocks

    Journal of Asian Earth Sciences

    (2011)
  • F.L. Liu et al.

    Zircon U–Pb ages, REE concentrations and HF isotope compositions of granitic leucosome and pegmatite from the north Sulu UHP terrane in China: constraints on the timing and nature of partial melting

    Lithos

    (2010)
  • D. London

    Granitic pegmatites: an assessment of current concepts and directions for the future

    Lithos

    (2005)
  • T. Pettke et al.

    Magmatic-to-hydrothermal crystallization in the W-Sn mineralized Mole Granite (NSW, Australia): part II: evolving zircon and thorite trace element chemistry

    Chemical Geology

    (2005)
  • D.B. Rowley et al.

    Ages of ultrahigh pressure metamorphism and protolith orthogneisses from the eastern Dabie Shan: U/Pb zircon geochronology

    Earth and Planetary Science Letters

    (1997)
  • D. Rubatto

    Zircon trace element geochemistry: partitioning with garnet and the link between U–Pb ages and metamorphism

    Chemical Geology

    (2002)
  • D. Rubatto et al.

    Zircon formation during fluid circulation in eclogites (Monviso, Western Alps): implications for Zr and Hf budget in subduction zones

    Geochimica et Cosmochimica Acta

    (2003)
  • E.W. Sawyer

    Migmatites formed by water-fluxed partial melting of a leucogranodiorite protolith: microstructures in the residual rocks and source of the fluid

    Lithos

    (2010)
  • Y.-M. Sheng et al.

    Fluid action on zircon growth and recrystallization during quartz veining within UHP eclogite: insights from U–Pb ages, O–Hf isotopes and trace elements

    Lithos

    (2012)
  • J.S. Stacey et al.

    Approximation of terrestrial lead isotope evolution by a two-stage model

    Earth and Planetary Science Letters

    (1975)
  • N.D. Tailby et al.

    Ti site occupancy in zircon

    Geochimica et Cosmochimica Acta

    (2011)
  • A. Audétat et al.

    Formation of a magmatic–hydrothermal ore deposit: insights with LA-ICP-MS analysis of fluid inclusions

    Science

    (1998)
  • F. Bea et al.

    A LA-ICP-MS evaluation of Zr reservoirs in common crustal rocks: implication for Zr and Hf geochemistry, and zircon-forming processes

    The Canadian Mineralogist

    (2006)
  • E.A. Belousova et al.

    Igneous zircon: trace element composition as an indicator of source rock type

    Contributions to Mineralogy and Petrology

    (2002)
  • B. Bingen et al.

    Trace element signature and U–Pb geochronology of eclogite-facies zircon, Bergen Arcs, Caledonides of Western Norway

    Contributions to Mineralogy and Petrology

    (2004)
  • M. Brown

    Melting of the continental crust during orogenesis: the thermal, rheological, and compositional consequences of melt transport from lower to upper continental crust

    Canadian Journal of Earth Sciences

    (2010)
  • T. Burri et al.

    Tertiary migmatites in the Central Alps: regional distribution, field relations, conditions of formation and tectonic implications

    Schweizerische Mineralogische Petrographische Mitteilungen

    (2005)
  • R.-X. Chen et al.

    Zircon U–Pb age and Hf isotope evidence for contrasting origin of bimodal protoliths for ultrahigh-pressure metamorphic rocks from the Chinese Continental Scientific Drilling project

    Journal of Metamorphic Geology

    (2007)
  • Y.-X. Chen et al.

    Petrological and zirconological evidence for anatexis of UHP quartzite during continental collision in the Sulu orogen

    Journal of Metamorphic Geology

    (2013)
  • D.J. Cherniak et al.

    Diffusion in zircon

    Reviews in Mineralogy and Geochemistry

    (2003)
  • J.D. Clemens

    Melting of the continental crust: fluid regimes, melting reactions, and source-rock fertility

  • B.L. Cong

    Ultrahigh-Pressure Metamorphic Rocks in the Dabieshan–Sulu Region of China

    (1996)
  • F. Corfu et al.

    Atlas of zircon textures

    Reviews in Mineralogy and Geochemistry

    (2003)
  • H. Degeling et al.

    Zr budgets for metamorphic reactions, and the formation of zircon from garnet breakdown

    Mineralogical Magazine

    (2001)
  • E.D.A. Ferriss et al.

    Computational study of the effect of pressure on the Ti-in-zircon geothermometer

    European Journal of Mineralogist

    (2008)
  • J.M. Ferry et al.

    New thermodynamic models and revised calibrations for the Ti-in-zircon and Zr-in-rutile thermometers

    Contributions to Mineralogy and Petrology

    (2007)
  • G. Fraser et al.

    Zirconium abundance in granulite-facies minerals, with implications for zircon geochronology in high-grade rocks

    Geology

    (1997)
  • X.-Y. Gao et al.

    Dehydration melting of ultrahigh-pressure eclogite in the Dabie orogen: evidence from multiphase solid inclusions in garnet

    Journal of Metamorphic Geology

    (2012)
  • T. Geisler et al.

    Re-equilibration of zircon in aqueous fluids and melts

    Elements

    (2007)
  • B. Gong et al.

    Geochronology and stable isotope geochemistry of UHP metamorphic rocks at Taohang in the Sulu orogen, east-central China

    International Geology Review

    (2007)
  • Cited by (0)

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