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Paleo-Asian Oceanic Slab Under the North China Craton Revealed by Carbonatites Derived from Subducted Limestones

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Paleo-Asian Oceanic Slab Under the North China Craton Revealed by Carbonatites Derived from Subducted Limestones Paleo-Asian oceanic slab under the North China craton revealed by carbonatites derived from subducted limestones Chunfei Chen1, Yongsheng Liu1*, Stephen F. Foley2, Mihai N. Ducea3,4, Detao He1, Zhaochu Hu1, Wei Chen1, and Keqing Zong1 1State Key Laboratory of Geological Processes and Mineral Resources, School of Earth Sciences, China University of Geosciences, Wuhan 430074, China 2ARC Centre of Excellence for Core to Crust Fluid Systems, Department of Earth and Planetary Sciences, Macquarie University, North Ryde, New South Wales 2109, Australia 3Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA 4Faculty of Geology and Geophysics, University of Bucharest, Bucharest, Romania ABSTRACT northward subduction of the Paleo-Tethys oce- It is widely accepted that the lithospheric mantle under the North China craton (NCC) anic and Yangtze plates in the Triassic, and has undergone comprehensive refertilization due to input from surrounding subducted slabs. (3) the westward subduction of the Pacific However, the possible contribution from the Paleo-Asian oceanic slab to the north is poorly plate since the Cretaceous (Fig. 1A) (Windley constrained, largely because of the lack of convincing evidence for the existence of this slab et al., 2010). Although all of these subduction under the NCC. We report here carbonatite intruding Neogene alkali basalts in the Hannuoba events could have contributed to widespread region, close to the northern margin of the NCC. Trace element patterns with positive Sr and lithospheric reactivation and thinning under U anomalies, negative high field strength elements (Nb, Ta, Zr, Hf, and Ti) and Ce anoma- the NCC (e.g., Griffin et al., 1998), the Pacific 87 86 18 lies, high Sr/ Sr ratios (0.70522-0.70796), and high d OSMOW (standard mean ocean water) plate subduction was generally advocated to values (22.2‰–23‰) indicate that this carbonatite had a limestone precursor. However, the account for the lithospheric thinning (Zhu et presence of coarse-grained mantle-derived clinopyroxene, orthopyroxene, and olivine, and al., 2011). The contribution of the PAOS and chemical features of the carbonates suggest that the carbonate melts were derived from the Paleo-Tethys oceanic slab to lithospheric thin- mantle. The carbonates have high 143Nd/144Nd ratios (0.51282-0.51298) and show negative ning remains enigmatic, largely because of the correlation between CaO and Ni contents, resulting from reaction between carbonate melt lack of convincing evidence for the existence of and peridotite. Considering the regional tectonic setting, the carbonatite probably formed these slabs under the NCC. If abundant carbon- by melting of subducted sedimentary carbonate rocks that formed part of the Paleo-Asian ate sediments were transported into deep mantle oceanic slab, and thus could provide the first direct evidence for the presence of the Paleo- during oceanic slab subduction, they could have Asian oceanic slab beneath the NCC. contributed fundamentally to the modification of the chemical and physical properties of the INTRODUCTION to the Earth’s surface via fluid (decarbonation lithospheric mantle by carbonate metasomatism. Carbonate platforms are common in oceanic reactions) and melt transport (Liu et al., 2015). In turn, subsequent mantle-derived melts (espe- realms. Small tropical oceans, exemplified by It has been suggested that the lithosphere of cially carbonatite) could contain the fingerprint many segments of the Tethys Ocean in Europe, the North China craton (NCC) was successively of subducted carbonate, which can be used to were covered with extensive carbonate plat- modified by (1) the southward subduction of the trace the origin of the subducted slab (Tappert et forms, found today in classic Alpine fold and Paleo-Asian oceanic slab (PAOS) between the al., 2009) and outline the crust-mantle recycling thrust belts. It is clear that some of these mate- early Paleozoic and the late Permian, (2) the of carbonate sediments. rials are subducted to mantle depths where they undergo decarbonation reactions and may be 110°E 120° 130° involved in partial melting (Collins et al., 2015; B CAOB ure A sut Doucelance et al., 2014; Ducea et al., 2005; ker lon alihu t Basalt Hammouda, 2003). Enormous budgets of CO at So D Carbonatite 2 Hannuoba rea u Faul magmatic arcs (Lee and Lackey, 2015) indepen- Banyan Obo L Ko 40°N Zhuolu Basalt an 2m dently require that carbonate is very influential Huairen T n in mass exchange at convergent margins. Sub- a C D duction magmatism is probably one of the most e Ol North China Craton oc c SC important tectonic mechanisms responsible for Ol ifi QDS c SC regulating the exchange of CO2 between the OB Earth’s interior and the atmosphere (McKenzie Pa CM 400km CM 1000µm 1000µm et al., 2016). Unfortunately, very few observa- 30° Yangtze Craton tional data exist to provide details on the mecha- nisms of transport of carbonate materials and Figure 1. A: Tectonic framework of the study area (modified from Windley et al., 2010). CAOB— Central Asian orogenic belt; QDSOB—Qinling-Dabie-Sulu orogenic belt. B: Field appearance CO from the surface to mantle depths and back 2 of the carbonatite intrusion. C: Residual olivine (Ol) xenocryst partly resorbed by carbonatite melt. SC—sparry calcite; CM—carbonate matrix. D: Monomineralic aggregates consist of *E-mail: [email protected] interlocking calcite grains. GEOLOGY, December 2016; v. 44; no. 12; p. 1039–1042 | Data Repository item 2016347 | doi:10.1130/G38365.1 | Published online XX Month 2016 GEOLOGY© 2016 Geological | Volume Society 44 | ofNumber America. 12 For | www.gsapubs.orgpermission to copy, contact [email protected]. 1039 4 Here we document a carbonatite intrusion MgO (0.8-4.8 wt%), and low alkali contents 10 Carbonatite intrusion with geochemical features of recycled limestone. (Na2O <0.01 wt% and K2O <0.05 wt%); many ACar The intrusion marks the subduction of an overly- are similar to limestones, and some are along a values 2 ing carbonate platform of the PAOS, to mantle mixing trend between limestone and peridotite 10 ed HR depths beneath the NCC, providing evidence (Fig. DR2). MgO contents in the carbonatites liz ZL ma AL for recycling of carbonate back to the Earth’s are higher than in limestones (0.13–1.65 wt%), 1 surface by buoyant diapirism and high-degree and show a negative correlation with CaO. Trace -nor melting. element patterns are similar to sedimentary PM 10-2 limestones with notably positive Sr and U anom- Rb Th Nb La Sr Zr Sm Ti Dy Lu SAMPLES, PETROLOGY, AND alies and negative high field strength element Ba U Ta Ce Nd Hf Eu Gd Yb GEOCHEMICAL COMPOSITIONS (HFSE; Zr, Hf, Nb, Ta, Ti) anomalies (Fig. 2). Samples were collected from a carbonatite The incompatible trace element contents of most Figure 2. Primitive mantle (PM) normalized trace element patterns for Hannuoba car- intrusion (0.5–3 m thick and >35 m wide) that samples are lower than average limestone, espe- bonatite intrusion compared to average intrudes Neogene basalts (22–10 Ma; Zhu, 1998) cially for Rb, Ba, and heavy rare earth elements carbonatite (ACarb) and average limestone at Hannuoba (eastern China; Fig. 1). The basalts (REEs) (Fig. 2). Their REE patterns show nega- (AL) (data sources are provided in the Data are distributed along the northern margin of the tive Ce anomalies (Ce/Ce* = 0.2–0.8) and posi- Repository [see footnote 1]). The Mesozoic Zhuolu (ZL) and Huairen (HR) carbonatite data NCC, and comprise intercalated tholeiitic, tran- tive Eu anomalies (Eu/Eu* = 1.15–2.98) (Table are from Yan et al. (2007), and primitive mantle sitional, and alkali basalts (Zhi et al., 1990). The DR2). The carbonate matrix contains 45.2-52.7 values are from McDonough and Sun (1995). source of the basalts probably contains abundant wt% CaO, 0.4-1.8 wt% MgO, and 1.5-5.0 wt% garnet-pyroxenite (Liu et al., 2008) or was meta- SiO2 (Table DR3). Both the carbonate matrix 13 somatized by carbonate-rich fluid (Dupuy et al., and phenocrysts have higher Ni contents (1-133 isotopic compositions [d CVPDB (Vienna Peedee 1992). The alkali basalts carry abundant granulite, ppm) than limestone, and show negative correla- belemnite) = -14.4‰ to -11.2‰] but heavy oxy- 18 pyroxenite, and peridotite xenoliths (Chen et al., tions between CaO and Ni (Fig. 3A). gen isotopic compositions [d OSMOW (SMOW— 2001). The evolved Sr-Nd isotopic compositions Carbonates in the carbonatite intrusion have standard mean ocean water) = 22.2‰ to 23‰] of some pyroxenite xenoliths indicate involve- higher 87Sr/86Sr (0.70522-0.70796) and slightly (Table DR2). ment of subducted sediments (e.g., Xu, 2002). higher 143Nd/144Nd (0.51282-0.51298) than typi- The carbonatite intrusion occurs in xenolith-free cal carbonatites (Fig. 3B). 143Nd/144Nd ratios show DISCUSSION basalt and has a sharp boundary with the basalt no correlation with Nd contents (0.4-2.8 ppm), layer. The overlying basalt is domed upward by whereas 87Sr/86Sr ratios correlate negatively with Mantle Derivation of the Carbonatite the invasion of the carbonatite melt (Fig. 1B). Sr contents (27-306 ppm) (Fig. DR3). Rare ara- Possible origins of the Ol and Cpx macro- The carbonatite contains aggregates of cal- gonite veinlets that crosscut one sample have crysts in the carbonatite intrusion are reaction cite phenocrysts and medium- to coarse-grained high Sr contents (2507–6600 ppm) and relatively products between carbonate melt and the basalt silicate macrocrysts set in a matrix of fine- low 87Sr/86Sr ratios (0.70411–0.70683) (Table wall rock (Jolis et al., 2013) or peridotite xeno- gained calcite (Figs. 1C and 1D), indicating DR4); the postmagmatic fluid that deposited crysts from the mantle that potentially provide rapid quenching. Monomineralic aggregates of this aragonite might have also partially modified records of the ascent process of the carbon- interlocking calcite grains (50–100 mm) indi- the sample (Fig.
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