Metamorphic Dehydration And Partial Melting In Continental Subduction Zones: A Study Of Low-T/UHP Metagranite In South Dabie

Fluid activity during crustal subduction and exhumation is very important to understand a wide spectrum of phenomena, including ultrahigh-pressure (UHP) metamorphism, syn-collisional magmatism and mineralization. It is also a key to understanding the evolution of the Earth, including the global circulation of water, the fate of deeply subducted slab, the origin of igneous rocks in collision orogenic belts, and the recycling of oceanic and continental crusts. UHP metamorphic rocks have been well recognized to occur in the Dabie-Sulu orogenic belt in east-central China, which is a sound target to study the nature and extent of fluid-rock interactions during pre-, syn- and post-peak UHP metamorphism during the continental collision. While much attention has been paid to fluid action in mafic HP to UHP eclogites in continental and oceanic subduction zones, less is focused on felsic UHP gneiss in continnetal subduction zones.This dissertation presents a combined study of petrography, whole-rock major and trace elements as well as Rb-Sr and Sm-Nd isotopes, mineral O isotopes and trace elements as well as zircon trace elements, U-Pb and Lu-Hf isotopes for two groups of low-T/UHP granitic gneiss in South Dabie orogen. The results demonstrate that metamorphic dehydration and partial melting occurred during subduction and exhumation of deeply subducted continental crust. As a progress in metamorphic zirconology, metamorphic growth and recrystallization of zircon can be distincted with respect to protolith inheritance, fluid and melt effects. Mechanisms of solid-state, replacement and dissolution recrystallization were involved in these reworking processes, manifesting the role of different types of metamorphic fluid (aqueous fluid, hydrous melt, and supercritical fluid). Metamorphic growth proceeds from aqueous fluid or hydrous melt during subduction and exhumation of deeply subducted continent. These results give a clue on not only the formation mechanism of metamorphic zircon, but also the action of different kinds of metamorphic fluid during continental collision.Petrographic observations show that Group I gneiss is lepidoblastic with a metasomatic relict texture. Many UHP metamorphic minerals such as garnet, epidote, phengitic muscovite and rutile remain residual skeletal or zoning textures. In contrast, Group II gneiss is leucogranitic with a granoblastic texture. Quartz and feldspar are major rock-forming minerals, and hydrous minetals such as epidote and muscovite are very limited. UHP metamorphic minerals such as (garnet, epidote and rutile) were not found in Group II gneiss, however, the content of K-feldspar is very high. Zircon U-Pb dating yields two groups of ages at 778±13 Ma and 223±4 Ma, respectively, corresponding to protolith formation in the middle Neoproterozoic and metamorphic modification in the Triassic. There are negativeεHf(t) values of -11.5±1.4 to -2.2±0.5 for the zircon, correspongding to old Hf model ages of 2.4 to 1.8 Ga. These are in contrast to those for UHP metaigneous rocks in Central Dabie that are dominated by positiveεHf(t) values of 1.1±0.6 to 8.2±0.7 and young Hf model ages of 1.0 to 1.3 Ga. Thus, there are two episodes of crustal growth in the late Mesoproterozoic (1.3 to 1.0 Ga) and the middle Paleoproterozoic (2.4 to 1.8 Ga), respectively, in the northern edge of the South China Block. Tectonic evolution from supercontinental rift to breakup is considered as a basic mechanism to cause the reworking of both ancient and juvenile crust at about 780 Ma.Zirconδ18O values of -2.8 to +4.7‰were obtained for the two groups of metagranites, which are lower than normal mantle values of 5.3±0.3‰. This is consistent with 18O depletion of metaigneous protolith due to high-T meteoric-hydrothermal alteration at Neoproterozoic. Most samples have extremely low 87Sr/86Sr ratios at t1 = 780 Ma, but very high 87Sr/86Sr ratios at t2 = 230 Ma. This suggests intensive fluid disturbance due to the hydrothermal alteration of protoliths during the Neoproterozoic magma emplacement and the metamorphic dehydration during the Triassic continental collision. The two groups of gneiss have similar patterns of REE and trace element partition. Group I gneiss displays good correlations between Nb and LREE but no correlations between Nb and LILE (Rb, Ba, Pb, Th and U), indicating differential mobilities of LILE due to dehydration. Together with the petrographic observation, metamorphic dehydration is evident in Group I gneiss during the prograde subduction. Thus the correlation between Nb and LREE is inherited from protolith rather than caused by metamorphic modification.For Group II gneiss, in contrast, Nb correlates with LILE, but not with LREE. This may indicate decoupling between the dehydration and LILE transport during continental collision. In addition, Group II gneiss has extremely low contents of FeO + MgO + TiO2 (1.04 to 2.08 wt.%), high SiO2 contents of 75.33 to 78.23 wt.%, and high total alkali (Na2O + K2O) contents (7.52 to 8.92 wt.%), comparable with compositions predicted from partial melting of felsic rocks by experimental studies. This indicates that the dehydration melting may have occurred in Group II gneiss due to breakdown of muscovite during"hot"exhumation. Almost no UHP metamorphic minerals survived; granoblastic texture occurs locally to result in a kind of metatexite migmatites due to dehydration melting without considerable escape of felsic melts from the host gneiss. The metamorphic ages for zircon from the two groups of gneiss are concordant with those for the transition from UHP to HP eclogite-facies recrystallization in the Dabie-Sulu orogenic belt. It also corresponds to the maximum temperature during the initial exhumation, which made fluid/melt available for the zircon growth.According to the metamorphic fluid involved, metamorphic zircon can be divided into five subtypes: metamorphic recrystallization via solid-state transformation, replacement alteration or dissolution reprecipitation, and metamorphic growth corresponds to chemical precipitation from aqueous fluid or hydrous melt. Different types of metamorphic zircon have their characteristic apparent 206Pb/238U ages, trace element compositions, Th/U ratios, 176Lu/177Hf and 176Hf/177Hf isotope ratios. Metamorphically grown zircons from the aqueous fluid or hydrous melt are characterized by concordant Triassic U-Pb ages, high contents of U, low Th/U and 176Lu/177Hf ratios, and elevated Hf isotope ratios. However, metamorphical zircon grown from the aqueous fluid has low contents of REE, Th and HFSE, whereas zircon grown from the hydrous melt show very high contents of REE, Th and HFSE.Metamorphic recrystallization results in varying degrees of reworking of protolith zircons, depending on the activity of aqueous fluid or hydrous melt during subduction-zone metamorphism. Metamorphic zircons formed by solid-state recrystallization exhibit the lowest degrees of reworking on internal textures and U-Pb isotopes, so that they are characterized by regular or blurred oscillatory zoing and discordant U-Pb ages between Neoproterozoic and Triassic. While the zircon internal texture and U-Pb isotope systems may be partially reworked by solid-state recrystallization, its initial Hf isotopes and trace elements keep unchanged with those for protolith zircons. This indicates that there is a lack of metamorphic fluid during the solid-state recrystallization.Replacement-recrystallized zircons show intenser metamorphic modification than solid-state recrystallization. They display fir-tree zoned, weak zoned or unzoned texture, and discordant U-Pb ages between Neoproterozoic and Triassic. Their REE patterns resemble those for protolith magmatic zircon except LREE enrichment for some zircons. This suggests that they underwent metamorphic reworking by replacement alteration in the presence of metamorphic fluid. The LREE enrichment may partly be caused by the presence of microscale LREE-bearing mineral inclusions (such as apatite, monazite or epidote) in the zircons. The 176Lu/177Hf ratios may decrease a little due to the garnet effect, but the 176Hf/177Hf ratios would remaine unchanged.The dissolution-recrystallized zircons display the highest degrees of modification by metamorphic fluid. They exhibit very weak cathodoluminescence, spongy or porous texture, with nearly concordant U-Pb ages if the metamorphic reworking by dissolution recrystallization is complete. Their trace elements show consistent enrichment of REE, Th, U and HFSE (Nb, Ta and Hf) relative to protolith zircon, with insignificant positive Ce anomalies in their REE patterns. Because the Hf isotope composition of dissolution recrystallized zircon is predominated by the protolith zircon, the fluid effect is insignificant on the change in the initial Hf isotope ratios. As a result, the 176Lu/177Hf and 176Hf/177Hf ratios keep almost unchanged relative to the protolith zircon.The consistent enrichment of trace elements in the sponge-textured zircons relative to the magmatic zircon indicates that a common hydrothermal fluid is not adequate for the replacement alteration because the common high-T/LP aqueous fluid cannot transfer a large amount of trace elements. Thus, the involvement of a special UHP metamorphic fluid such as supercritical fluid or hydrous melt is required that has a strong capacity to extract significant amounts of LREE, HREE Th, U and HFSE from such accessory minerals as allanite, garnet, rutile and zircon. Because these minerals are stable in the field of hydrous melt in granite-water systems, they are not able to be decomposed during the exhumation of deeply subducted continental crust. Instead, the supercritical fluid is suggested to transport the LREE, HREE, Th, U and HFSE in the accessory minerals to recrystallized zircons. While the supercritical fluid is stable in deep subduction zones, it would separate into a hydrous melt and an aqueous fluid due to abrupt depression and continuous heating during the initial"hot"exhumation. As a result, the dissolved components would exsolve from the supercritical fluid, with significant incorporation of the fluid-immobile elements into precipitating minerals. This is indicated by occurrence of vein aggregate of REE-rich accessory minerals (epidote, titanite and zircon) in the fissure of exhumated metagranites. Therefore, the action of supercritical fluid is evident under the low-T/UHP metamorphic conditions.