Nd isotope re-equilibration during high temperature metamorphism across an orogenic belt: Evidence from monazite and garnet

doi:10.1016/j.chemgeo.2020.119751

Chemical Geology

Volume 551, 30 September 2020, 119751

https://doi.org/10.1016/j.chemgeo.2020.119751 Get rights and content

Highlights

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Multi-chronometer constraints on the timing of JLJB metamorphism
•
Homogenous initial Nd isotope compositions between bulk-rock and minerals
•
Monazite melting and crystallization from anatectic melt during high-T metamorphism
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Nd isotope re-equilibration suggests Sm-Nd mobility during tectonothermal processes.

Abstract

Monazite is a powerful U-Th-Pb geochronometer commonly used to determine the tempo of orogenic processes. As a light rare earth element (LREE)-enriched mineral, monazite also records key information for understanding the behavior of Sm-Nd isotope systematics during tectonothermal events. However, Sm-Nd isotope studies of monazite have received far less attention than its use as a U-Pb geochronometer. Here we investigate coupled U-Pb ages and Sm-Nd isotopic compositions of monazite from partially melted metasedimentary rocks in the Jiao-Liao-Ji orogenic belt (JLJB), North China craton, to provide insight into Sm-Nd isotope behavior during high-temperature metamorphism and crustal melting. We utilize several complementary dating systems in the same sample, including garnet Lu-Hf and Sm-Nd, zircon U-Pb, and monazite U-Pb geochronology, to gain precise time constraints on the P-T evolution of the JLJB. Garnet Lu-Hf isochron dates of 1.97–1.94 Ga are interpreted to represent prograde garnet growth, consistent with the prograde metamorphic zircon dates of 1.96–1.90 Ga; garnet Sm-Nd dates agree with retrograde metamorphic zircon dates, and all monazite U-Pb dates define a relatively narrow age range of 1.86–1.83 Ga, interpreted to represent retrograde cooling. The lack of older monazite grains matching zircon or garnet dates suggests the dissolution of pre-existing monazites into partial melt. Monazite in all samples, irrespective of occurring as garnet inclusions or in the matrix, yields homogenous Nd isotope compositions (ε_Nd ~ −5) at ~1.85 Ga, which are consistent with the garnet and bulk-rock values. Thus, Nd isotope re-equilibration between bulk-rock and minerals occurs synchronously across the orogenic belt during high-T metamorphism (~750 °C) where melting was involved, with no evidence of a pre-metamorphic isotope signature. This suggests mobility in the Sm-Nd isotope system during high-grade metamorphism and melting, where the Nd isotope signature of monazite likely represents the most recent tectonothermal event.

Introduction

Monazite is a common light rare earth element (LREE)-rich accessory phase that has been widely utilized as a U-Th-Pb geochronometer in metamorphic rocks (e.g., Heaman and Parrish, 1991; Foster et al., 2002; Kohn et al., 2005), which provides important timing constraints on the rates of subduction zone processes during orogenesis. This is often achieved by linking the age (t) of minerals to the pressure (P) and temperature (T) conditions of metamorphism (e.g., Rubatto, 2002; Kohn et al., 2005; Kelsey et al., 2008; Reno et al., 2012; Yakymchuk et al., 2015; Warren et al., 2019). Although in-situ determination of U-Pb dates in monazite is generally precise and efficient to obtain, the interpretation of these dates may not be clear unless they can be linked to a rock-forming mineral proxy that represents certain P-T conditions. For this reason, monazite and garnet in the same metamorphic rock from orogenic belts are often paired (e.g., Hermann and Rubatto, 2003; Stearns et al., 2013; Rubatto et al., 2013; Holder et al., 2015). However, as we show in this study, correlation between age and elemental variation in monazite may not provide sufficient information to interpret P-T-t evolution in metamorphic rocks that have undergone high-T events where crustal melting was involved.

Time constraints have also been obtained from the U-Th-Pb dates of monazite inclusions in garnet (e.g., Foster et al., 2000; Mottram et al., 2014). Garnet is thought to armor the inherited monazite inclusions from modification by later metamorphic events, thereby pre-dating garnet growth (e.g., Zhou and Hensen, 1995; Foster et al., 2000). In some cases, episodic monazite formation before and after garnet growth can be determined by the occurrence of monazite (e.g., older inclusions vs. younger matrix grains; Foster et al., 2000). However, other studies of monazite inclusions in garnet have suggested more complicated scenarios, with some monazite inclusions being younger or concurrent in age with matrix monazite (e.g., Martin et al., 2007; Hoisch et al., 2008). These observations have been interpreted as dissolution, recrystallization, and intergrowth of younger monazite inclusions, facilitated by the fractures in garnet that connect with the matrix (Zhu et al., 1997; Martin et al., 2007). Although more recent studies have suggested the possibility of growth of monazite inclusions during garnet break-down (e.g., Yakymchuk et al., 2015), or resulting from isotopic resetting under tectonic strain (e.g., Hagen-Peter et al., 2016), the mechanisms that form younger monazite inclusions in garnet remain largely unclear.

Along with being U-Pb chronometers, LREE-rich accessory minerals (e.g., monazite, titanite, apatite) also have utility as Nd isotope repositories, which have gained increasing attention recently (e.g., Fisher et al., 2011; Hammerli et al., 2014; Fisher et al., 2017; Spandler et al., 2018). In igneous rocks, combined U-Pb age, trace-element, and Sm-Nd isotope characteristics of monazite, for example, have been used to constrain open-system processes of magma interacting with complex sources (Fisher et al., 2017). In metamorphic rocks, traditional petrochronology of monazite and titanite uses trace elemental chemistry to better interpret the corresponding U-Pb ages (e.g., Kohn et al., 2005; Rubatto et al., 2013; Holder et al., 2015; Scibiorski et al., 2019). As these minerals also serve as the major carriers of Sm and Nd in their host rocks, they are potentially useful in gaining insight into isotopic behavior during metamorphic processes. However, recent studies have demonstrated the need for a better understanding of the behavior of Sm-Nd isotope systematics of accessory minerals during tectonothermal events. Nd isotope disequilibrium during crustal anatexis has been suggested to be the result of variable REE-rich phases contributing to the partial melting reactions (Zeng et al., 2005a, Zeng et al., 2005b; Perini et al., 2009). Hammerli et al. (2014) and Spandler et al. (2018) have examined Nd isotope homogenization between bulk-rock and minerals during metamorphism by studying REE-rich minerals from subsolidus metapelites under various P-T conditions. Further, complex-wide re-equilibration of Nd has been suggested to have occurred in the Lewisian complex of northwest Scotland (Whitehouse, 1988), the Itsaq Gneiss of southern West Greenland, and Acasta Gneiss Complex of Canada (Moorbath et al., 1997). Recent work on these two Eoarchean complexes has demonstrated re-equilibration of Nd isotope compositions of apatite and titanite (major repositories of Nd) during multiple tectonothermal events, pointing to the potential for Sm-Nd modification resulting from post-crystallization processes (e.g., Hammerli et al., 2019; Fisher et al., 2019).

In this paper, we focus our attention on the Jiao-Liao-Ji orogenic belt (JLJB), located on the eastern edge of the North China craton (NCC), where P-T conditions of metamorphic rocks have been well-established (e.g., Tam et al., 2011; Cai et al., 2017; P.-H. Liu et al., 2017; and references therein). In order to understand the behavior of monazite U-Pb and Sm-Nd isotope systems during high temperature metamorphism followed by slow exhumation, we first employ four different dating systems for the same rock, including garnet Lu-Hf and Sm-Nd geochronology, as well as zircon and monazite U-Pb geochronology, to gain better time constraints on the metamorphic history. The integration of complementary chronometers improves and refines the existing metamorphic geochronology in the JLJB that was primarily established using zircon U-Pb dates. Using this geochronology as a foundation, our new Sm-Nd isotope results of monazite, garnet, and bulk-rocks not only document isotope re-equilibration during high-T metamorphism, but also shed new light on the mobility of Nd in monazite from partially melted rocks. From this, we suggest both U-Pb and Sm-Nd systems of monazite record only the last closure in a tectonothermal event.

Section snippets

Geologic background and samples

The Archean-Paleoproterozoic North China craton (NCC) is divided into Western and Eastern blocks by the Trans-North China orogen (TNCO, Fig. 1; Zhao et al., 2012). Within the eastern block of NCC, two sub-blocks—the Longgang block in the northwest and Nangrim block in the southeast (Fig. 1)—are divided by the Jiao-Liao-Ji orogenic belt (JLJB). The JLJB is a NE-SW trending zone that covers an area of ~1200 km length and 100–200 km width in northeastern China (Zhao et al., 2012). The belt mainly

Garnet Lu-Hf and Sm-Nd geochronology

Two bulk-rock (powder) and multiple garnet fractions (~250 mg aliquots) from each sample were processed in the Radiogenic Isotope and Geochronology Laboratory (RIGL) at Washington State University (WSU). Garnet fractions were selected to be as free of visible inclusions as possible and were dissolved using the partial dissolution approach (e.g., Wang et al., 2019), with the aim of leaving zircons undigested, thereby avoiding the effects of inherited zircons from the protolith. This approach is

Garnet and monazite compositional maps

Representative X-ray maps of major element (Ca, Mn, Fe) compositions of garnets are shown in Fig. 2; the Th and Y compositions of monazites are shown in Fig. 3 (more grains are shown in Supplementary Fig. S2). Both garnets and monazites (within garnet, as well as in the matrix) were mapped in thin section from two samples (15SD29 and 15SD30). These maps were used to help interpret the garnet ages, and to guide the in-situ laser ablation analyses of monazites. No obvious compositional zoning of

Garnet age

Perhaps the most important factor to consider when interpreting garnet Lu-Hf and Sm-Nd dates is when a respective isotope system is closed, relative to the timing of peak metamorphic temperature. Therefore, knowledge of major element zoning, as well as the distribution of elements of interest (Lu, Hf, Sm, Nd) in garnet, and the temperatures of metamorphism, are crucial for interpreting garnet dates. The garnet Lu profiles generally show a central peak with satellite peaks toward the garnet rims

Conclusions

This study reports U-Pb ages and Sm-Nd isotope compositions of monazite—and their corresponding bulk rock—of meta-sedimentary rocks from the Jiao-Liao-Ji orogenic belt in the North China craton. These data are integrated with zircon U-Pb and garnet Lu-Hf and Sm-Nd geochronology to gain a more complete understanding of the metamorphic history. These results also provide new insights into Sm-Nd isotope systemics in monazite during high-T metamorphism.

1.
The JLJB rocks have monazite U-Pb dates of

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This study was benefited from instrumentation acquired with NSF grant MRI-1626670 to Vervoort; Field work and sample processing are supported by NNSFC grant 41872224 and CAGS Research Fund YYWF201708 to Cao. We thank S. Boroughs, C. Knaack, D. Wilford, and C. Zhang from WSU for their help with electron microprobe and the RIGL clean lab. We are also thankful to E. Bullock from EPL at Carnegie Science for the help with microprobe imaging. Constructive comments from M. Whitehouse and an anonymous

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Sm-Nd and U-Pb isotope behavior of REE-rich accessory minerals in pegmatite during overprinted metamorphic and hydrothermal events: Evidence from the Paleoproterozoic rare-earth pegmatite in the lesser Qinling district of China
2023, Precambrian Research
Sm-Nd and U-Pb isotope systems are widely used to dating various magmatism and tracing geological processes, but their behaviors during post-magmatic metamorphic and/or hydrothermal disturbances remain unclear. Here, we investigated the trace elements, U-Pb and Sm-Nd isotope systems of accessory minerals (allanite, titanite, and apatite) from the Huangjiagou Paleoproterozoic rare-earth pegmatite, to determine the timing of magmatism and also the overprinted metamorphic and hydrothermal events, with a particular emphasis on the isotopic behavior of these accessory minerals under metamorphic event and fluid-rock reaction. Two-period accessory minerals are identified: Group I allanite, crystallized in the magmatic period but was modified by high-temperature metamorphism and hydrothermal metasomatism; and Group II allanite, titanite and apatite, which newly precipitated in the late hydrothermal event. In-situ U-Pb dating of the Group I allanite (type Ia) yielded a magmatic age of ca. 1.82 Ga, whereas the type Ib allanite record the high-temperature anhydrous thermal metamorphism at ca. 1.75 Ga. Group II allanite and titanite yielded U-Pb ages of ca. 131 Ma, indicating a Mesozoic hydrothermal metasomatism overprinting, due to the emplacement of neighboring granites. Group I allanite exhibits different zones and inconsistent U-Pb ages, but they exhibit an identical Sm-Nd isochron age (∼1.83 Ga) broadly consistent with the pegmatite age, indicating immobility nature of the Sm-Nd system. Group II titanite and allanite exhibit a Sm-Nd isochron age of ∼130 Ma with initial ε_Nd(t) values of −27.5 to −29.8, consistent with the whole-rock ε_Nd(t = 130 Ma) values of granitic rocks, implying a total redistribution of the REEs. While, Group II apatite and coexisting allanite did not produce geologic meaningful Sm-Nd ages with a wider initial ε_Nd(t) range of −15.8 to –23.7, indicating the fluid has incompletely equilibrated Sm-Nd isotopic system due to various fluid-rock reaction. Our study provides a typical example to illustrate the robustness of the Sm-Nd isotopic system of magmatic allanite under the disturbance of later metamorphism and metasomatism, and shows the different degree equilibration of Sm-Nd isotopes in hydrothermal systems under various fluid-rock reactions.
Geochemical evidence for partial melting of progressively varied crustal sources for leucogranites during the Oligocene–Miocene in the Himalayan orogen
2022, Chemical Geology
Citation Excerpt :
However, recent studies by in-situ analyses of these accessory minerals in metasedimentary rocks show that blueschist to eclogite facies metamorphism at low geothermal gradients cannot modify the inherited radiogenic isotope components (e.g., Spandler et al., 2018). In contrast, amphibolite to granulite facies metamorphism at high geothermal gradients can result in Nd isotope re-equilibrium between accessory minerals and whole-rock (e.g., Hammerli et al., 2014; Wang et al., 2020). These observations indicate that the existence of inherited radiogenic isotope components before the crustal anatexis may not always be the case.
Crustal anatexis in collisional orogens has great bearing on geochemical differentiation of the continental crust. However, it is often uncertain what kind of crustal rocks were partially melted for felsic magmatism at convergent plate boundaries. To address this issue, a combined study of in-situ monazite U–Th–Pb ages, in-situ monazite and allanite SmNd isotopes and whole-rock SmNd isotopes was carried out for Higher Himalayan metamorphic rocks and leucogranites in the Himalayan orogen. Metapelite, metagreywacke and granitic gneiss are the dominant constituents of the Higher Himalayan Crystallines, and they experienced upper amphibolite to granulite facies metamorphism at ca. 26–13 Ma. Although the three rock types show consistently negative ε_Nd(t) values at t = 20 Ma, their Nd isotope compositions become less and less enriched from metapelite through metagreywacke to granitic gneiss. The metapelite has the lowest ε_Nd(t) values of −19.9 to −15.7, the metagreywacke has intermediate ε_Nd(t) values of −17.4 to −12.7, and the granitic gneiss has the highest ε_Nd(t) values of −14.1 to −7.7. Leucogranitic magmatism occurred at ages from ca. 26 to 7 Ma, coeval with anatectic metamorphism of the three rock types during the Oligocene–Miocene. Relict zircon UPb age distributions, whole-rock trace element patterns, initial Nd isotope compositions and two-stage Nd model ages for the Higher Himalayan leucogranites are comparable with those for the Higher Himalayan metamorphic rocks, confirming that the metapelite, metagreywacke and granitic gneiss would have their compositional counterparts at structurally deeper positions to serve as the crustal sources of the leucogranites. This is also verified by forward phase equilibrium modelling for partial melting of the metamorphic rocks with respect to the differences in their composition and fertility. In addition, the leucogranites show a decreasing trend in ε_Nd(t) values with their ages, indicating that the dominant crustal sources would gradually change from the least fertile granitic gneiss through the intermediately fertile metagreywacke to the most fertile metapelite during the protracted crustal anatexis from the late Oligocene to the Miocene. Therefore, the deep crust in the Himalayan orogen would consist of the metamorphic rocks with similar compositions to the shallow crust. The combined geochronological and geochemical study of accessory minerals and host rocks can provide the genetic link between the crustal sources and their melting products in collisional orogens. Nevertheless, the Nd isotope variation in these leucogranites may also be related to incongruent melting of the crustal sources, which has a potential to result in the Nd isotope disequilibrium between melt and residue.

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