Yuan-Bao Wu

Genesis of zircon and its constraints on interpretation of U-Pb age

Zircon U-Pb dating is the most commonly used method for isotopic geochronology. However, it has been a difficult issue when relating zircon U-Pb ages to metamorphic conditions in complex metamorphic rocks. Much progress has been made in the past decade with respect to the genesis of zircon and its constraints on interpretation of U-Pb age. Three methods have been proposed to link zircon U-Pb age to metamorphic conditions: (i) internal structure; (ii) trace element feature; (iii) mineral inclusion composition. Magmatic zircon shows typical oscillatory zoning and/or sector zoning, whereas metamorphic zircon has internal structures such as no zoned, weakly zoned, cloudy zoned, sector zoned, planar zoned, and patched zoned ones. Zircons formed in different geological environments generally have characteristic internal structures. Magmatic zircons from different rock types have variable trace element abundances, with a general trend of increasing trace element abundances in zircons from ultramafic through mafic to granitic rocks. Zircons formed under different metamorphic conditions have different trace element characteristics that can be used to relate their formation to metamorphic conditions. It is an effective way to relate zircon growth to certain P-T conditions by studying the trace element partitioning between coexisting metamorphic zircon and garnet in high-grade metamorphic rocks containing both zircon and garnet. Primary mineral inclusions in zircon can also provide unambiguous constraints on its formation conditions. Therefore, interpretation of zircon U-Pb ages can be constrained by its internal structure, trace element composition, mineral inclusion and so on.

Low-Grade Metamorphic Rocks in the Dabie-Sulu Orogenic Belt: A Passive-Margin Accretionary Wedge Deformed during Continent Subduction

Greenschist-facies metasedimentary and meta-igneous rocks occur continuously along the northern margin, and sporadically in the interior, of the Dabie-Sulu orogenic belt in east-central China. An integrated study of geochronological, petrological, and paleontological observations demonstrates that precursors of flysch-facies metasedimentary rocks were deposited along the northern, passive continental margin of the Yangtze plate prior to the Triassic, and that protoliths of the meta-igneous rocks are a product of Middle Neoproterozoic bimodal magmatism along the northern margin of this plate. Except for the striking contrast in metamorphic grade, these low-grade rocks generally can be correlated in protolith origin and age with ultrahigh-pressure metamorphic rocks within the orogenic belt. Relationships in time and space between these rocks of contrasting grades can be reasonably interpreted through an accretionary wedge model that links their evolution with continent subduction. The low-grade metamorphic rocks of the subducting accretionary wedge consist of two parts: (1) large masses of metasedimentary rocks (including slates, schists, phyllites, metasandstones, and marble) along the northern margin of the Dabie-Sulu orogenic belt and deformed igneous rocks of Middle Neoproterozoic age; (2) sporadic outcrops in the interior of the belt of metavolcanics, metaclastics, phyllite, and marbles. During Triassic subduction of the Yangtze plate, the sedimentary cover and its underlying basement were partly scraped off by the overthrusted North China plate. The scraped-off materials accumulated in front of the overriding plate, forming an accretionary wedge that underwent deformation and metamorphism under greenschist-facies conditions. The present study also provides a constraint on the location of the Triassic suture zone between the North China and Yangtze plates. It is located below, or north of, the accretionary wedge (i.e., the Beihuaiyang zone in the Dabie region and the Wulian-Penglai zone in the Sulu region) rather than along the northern margin of the ultrahigh-pressure metamorphic zones.

Zircon isotope evidence for recycling of subducted continental crust in post‐collisional granitoids from the Dabie terrane in China

[1]xa0Recycling of subducted continental crust in continental collision belts is examined by a combined study of zircon U-Pb dating and O isotope analysis of mineral separates from post-collisional granitoids in the Dabie terrane, east-central China. Although zircon U-Pb dating give concordant ages of 121 to 131 Ma for magma crystallization, the occurrence of inherited cores is identified by CL imaging and SHRIMP dating in some zircon grains that yield older ages of 742 to 815 Ma and 222 Ma, respectively, in agreement with Neoproterozoic protolith ages of UHP metaigneous rocks and Triassic tectono-metamorphic event in the Dabie-Sulu orogen. All the samples have zircon δ18O values of 4.14 to 6.11‰, most of which differ from the δ18O value of 5.3±0.3‰ for the normal mantle zircon. This indicates the remelting of continental crust containing both 18O-depleted and enriched components, with significant isotope homogenization relative to the UHP metaigneous rocks. These observations indicate the involvement of the subducted Yangtze plate in the Cretaceous granitoids and thus provide evidence for geochemical recycling of the continental crust that was thickened by the Triassic collision.

Zircon U-Pb age and o isotope evidence for neoproterozoic low-18O magmatism during supercontinental rifting in South China: Implications for the snowball earth event

Identification of Neoproterozoic low δ18O igneous rocks is a key to demonstrate that high-T water-rock interaction and low-18O magmatism in rift tectonic zones can serve as an efficient interface to transport heat and material from the Earths interior to exterior at the onset of the snowball Earth event. Low δ18O values of −10.9 to +4.4 permil characteristic of meteoric origin were observed, on an areal extent over 30,000 km2, for mid-Neoproterozoic metaigneous zircons from the ultrahigh-pressure eclogite-facies metamorphic belt of Triassic ages along the Dabie-Sulu orogenic belt, China. Although the zircon O isotope system could be reset to acquire its low δ18O values by exchange with matrix minerals during “wet” eclogite-facies metamorphism, the abnormal 18O depletion is confirmed by occurrence of Neoproterozoic low δ18O zircons in low-grade metaigneous rocks in the same belt. Cathodoluminscence imaging of zircons from low-grade metagranitoid and metavolcanic rocks shows preservation of the internal structures of oscillatory zoning typical of magmatic origin. Hornblende from two granites of low δ18O zircons gave Ar-Ar plateau ages of 747 ± 6 and 776 ± 12 Ma, ruling out that Phanerozoic metamorphism effected mineral O isotope values. Two groups of U-Pb ages were measured at 756 ± 5 Ma and 782 ± 5 Ma, respectively, for the low δ18O zircons. This indicates that low δ18O zircons crystallized during two episodes of low-18O magmatism in response to tectonic evolution from supercontinental rifting at ∼780 Ma to breakup at ∼750 Ma. The second episode of low-18O magmatism is contemporaneous with the continental Kaigas iceage. A plate-rift model is advanced to account for the tectonic connection between the low-18O magmatism, the continental glaciation and the supercontinental rifting during the Cryogenian period. High-T meteoric water-rock interaction occurred prior to low-18O magmatism in rift tectonic zones, and caldera collapse was responsible for melting of hydrothermally altered low δ18O rocks during rifting of the South China Block from the Rodinia supercontinent.

Neoproterozoic granitoid in northwest Sulu and its bearing on the North China‐South China Blocks boundary in east China

[1]xa0Middle Neoproterozoic granitoid rocks are found in the Wulian region that is located in the northwestern part of the Sulu orogen and is approximately bounded by the Wulian-Yantai fault in southeast. Zircon U-Pb dating yields concordant ages of 738 ± 10 to 758 ± 5 Ma for three granites and one gabbro; hornblende Ar-Ar dating on the gabbro gives consistent plateau and isochron ages of 719.1 ± 3.7 and 717.9 ± 7.2 Ma, respectively. This suggests that the Wulian granitoids correspond to bimodal magmatism during the mid-Neoproterozoic in the northern margin of the South China Block in response to the breakup of Rodinia supercontinent. They were tectonic slices scraped off from the upper part of the South China Block during the Triassic continental subduction. This demonstrates that the suture location between the North China Block and the South China Block lies north of the Wulian and Penglai Groups rather than along the Wulian-Yantai fault that marks the northern margin of ultrahigh pressure metamorphic belt.

Zircon U-Pb ages for Wulian granites in northwest Sulu and their tectonic implications

The Wulian complex occurring in the north-western part of the Sulu orogen consists of metaigneous and metasedimentary rocks at greenschist-facies. Zircon U-Pb dating for three granite intrusions within it yields concordant ages of (672±4), (742±9) and (747±14) Ma, respectively. These ages indicate that the granites are the Neoproterozoic intrusions in the northern margin of the Yangtze plate, and correspond to the magmatic complex at Luzhenguan in the eastern part of the Beihuaiyang belt in the Dabie orogen. They were tectonic slices scraped off from the Yangtze plate during Triassic subduction and thus belong to a part of the accretionary wedge of the Yangtze plate subduction. The discovery of extensive Neoproterozoic magmatic rocks in the Wulian area suggests that the suture location between the North China and the Yangtze plates lies north of the Wulian complex.

Geochronology and Stable Isotope Geochemistry of UHP Metamorphic Rocks at Taohang in the Sulu Orogen, East-Central China

Zircon U-Pb dating, mineral Sm-Nd isochron dating, and O and H isotope analyses were carried out for ultrahigh-pressure (UHP) eclogite and granitic gneiss from Taohang in the Sulu orogen. Besides heterogeneous 18O depletion on an outcrop scale, mineral-pair O isotope thermometry indicates that refractory garnet and zircon attained and preserved equilibrium fractionations at about 820 to 560°C under eclogite-facies conditions. Zircons from the UHP metamorphic rocks have low δ18O values of -1.3 to 4.2‰, variably lower than δ18O values of 5.3 ± 0.3‰ for normal mantle zircons. U-Pb discordia dating of 18O-depleted zircons yields a protolith age of 770 ± 23 Ma and a metamorphic age of 214 ± 9 Ma. Therefore, the 18O-depleted zircons crystallized from a mid-Neoproterozoic low-18O magma whose precursor experienced high-T meteoric-hydrothermal alteration prior to melting in an active rifting zone. Both H isotope composition and H2O concentration were measured by the TCEA-MS online technique. The results show δD values of -121 to -58‰ for nominally anhydrous minerals and -101 to -62‰ for hydroxyl-bearing minerals, consistent with incorporation of meteoric water into protoliths of UHP meta-igneous rocks by high-T alteration and remelting. Hundreds to thousands of ppm H2O were detected in the forms of both molecular water and structural hydroxyl to be present in the nominally anhydrous minerals, providing an important budget of water content (besides hydrous minerals) in deeply subducted continental crust. A Gt-Wr-Pl Sm-Nd isochron age of 214 ± 10 Ma was obtained, in agreement with the zircon U-Pb age and corresponding to the state of O isotope equilibrium between the isochron minerals. Thus both ages are interpreted to represent the time of high-pressure eclogite-facies recrystallization during the initial exhumation. A fluid-present process for zircon overgrowth and Nd-O isotopic reequilibration is evident for this episode of retrogression. On the other hand, a Gt-Kfs Sm-Nd isochron age of 164 ± 11 Ma was obtained, corresponding to the state of O isotope disequilibrium between garnet and K-feldspar. This age postdates the Triassic collision orogeny, and thus has no relevance to the processes of both continental subduction and exhumation, suggesting limited fluid activity in the post-collisional stage. Therefore, the state of O isotope equilibrium or disequilibrium between coexisting minerals in high-grade metamorphic rocks provides a direct test for the validity of the mineral Sm-Nd chronometer in either case.

Anomalously high δD values in the mantle

[1]xa0Hydrogen isotopic compositions of four amphibole grains from three lherzolite xenoliths hosted in Cenozoic basanites of Nushan, eastern China have been analyzed by ion microprobe. δD values of all analyzed points range from −94 to +46‰. Large heterogeneities of D/H ratios within single grains have been observed with variations of δD up to 80‰ on the scale of less than 400 μm. Neither correlation between δD values and water contents of the analyzed points nor regular variation between δD values and positions of analyzed points within the same grain can be found, implying that the scatter of δD values could not result from a late shallow process such as hydrogen loss or hydrothermal alterations and should be considered as inherited from the source at depth. Chemical compositions of Nushan amphiboles are very homogeneous, excluding the possibility that the scatter of δD values could arise from variable fractionation factors between a single fluid source and minerals. Therefore, metasomatic fluids responsible for the formation of Nushan amphiboles should have variable and unusually high hydrogen isotopic compositions. We suggest that this variation in the metasomatic fluids could be related to melt degassing in the mantle source.

Zircon SIMS ages and chemical compositions from Northern Dabie Terrain: Its implication for pyroxenite genesis

We present the results of a detailed micro-scale investigation of zircons from pyroxenites, Daoshicong Northern Dabie using a combination of SIMS and ICPMS. The SIMS measurements gave ages of 134–159 Ma. Its average of (144.5±6.2) Ma is interpreted as the best estimate of the pyroxenite intrusion. The crystallization of zircons continued for quite a long time and underwent slow cooling. The pyroxenites are products of post-collision magmatism. The REE pattern is HREE-enriched, and its HREE concentrations fall between the magmatic and metamorphic range of gneissic zircons from the Dabie area, which indicate the involvement of crust material in its mantle source.

Anomalously high δD and micro-scale hydrogen isotope heterogeneities in the mantle: Ion microprobe analysis of amiboles from peridotite xenoliths at Nushan, eastern China

Hydrogen isotopic compositions of four amphibole grains from three pieces of lherzolite xenoliths in Cenozoic basanites of Nushan, eastern China have been analyzed by ion microprobe. δD values of all analyzed points range from −94‰ to +46‰, some of which are much higher than the highest δD (+8‰) reported previously for mantle materials. The heterogeneities of D/H ratios within single grains have been observed, the variation of δD is up to 80‰ on the scale of less than 400 μm. No correlation between hydrogen isotopic ratios and hydrogen contents can be found, implying that the scatter of δD values could not result from a late shallow process such as hydrogen loss or hydrothermal alterations and should be considered as inherited from the source at depth. Chemical compositions of Nushan amphiboles are very homogeneous, excluding the fact that the scatter of δD values could arise from variable fractionation factors between a single fluid source and minerals. Therefore, metasomatic fluids responsible for the formation of Nushan amphiboles should be heterogeneous and result in the observed large variable and anomalously high δD values of amphiboles. We suggested that such metasomatic fluids could be related to magma degassing in the mantle source. Based on the D-H diffusion data and the scale of hydrogen isotope heterogeneities, it was inferred that the mantle metasomatism took place soon before the eruption of host magma.