Zirconological tracing of transition between aqueous fluid and hydrous melt in the crust: Constraints from pegmatite vein and host gneiss in the Sulu orogen
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)
Correction of common lead in U–Pb analyses that do not report 204Pb
Chemical Geology
(2002)- et al.
Zircon geochronology of migmatite gneisses along the Mylonite Zone (Sweden): a major Sveconorwegian terrane boundary in the Baltic Shield
Precambrian Research
(2002) - et al.
The magmatic–hydrothermal evolution of two barren granites: a melt and fluid inclusion study of the Rito del Medio and Canada Pinabete plutons in northern New Mexico (USA)
Geochimica et Cosmochimica Acta
(2003) - et al.
Behavior of accessory phases and redistribution of Zr, REE, Y, Th, and U during metamorphism and partial melting of metapelites in the lower crust: an example from the Kinzigite Formation of Ivrea–Verbano, NW Italy
Geochimica et Cosmochimica Acta
(1999) - et al.
Tectonically controlled fluid flow and water-assisted melting in the middle crust: an example from the Central Alps
Lithos
(2008) - et al.
Origin of retrograde fluid in ultrahigh-pressure metamorphic rocks: constraints from mineral hydrogen isotope and water content changes in eclogite-gneiss transitions in the Sulu orogen
Geochimica et Cosmochimica Acta
(2007) - et al.
Oxygen isotope geochemistry of ultrahigh-pressure metamorphic rocks from 200–4000 m core samples of the Chinese Continental Scientific Drilling
Chemical Geology
(2007) - et al.
Metamorphic growth and recrystallization of zircon: distinction by simultaneous in-situ analyses of trace elements, U–Th–Pb and Lu-Hf isotopes in zircon from eclogite-facies rocks in the Sulu orogen
Lithos
(2010) - et al.
Metamorphic growth and recrystallization of zircons in extremely 18O-depleted rocks during eclogite-facies metamorphism: evidence from U–Pb ages, trace elements, and O–Hf isotopes
Geochimica et Cosmochimica Acta
(2011) - et al.
Mineral hydrogen isotopes and water contents in ultrahigh-pressure metabasite and metagranite: constraints on fluid flow during continental subduction-zone metamorphism
Chemical Geology
(2011)