November 2022 LIP of the Month

The Cambrian-Ordovician Pinghe silicic large igneous province

Wei Dan^{1, 2}, J. Brendan Murphy^3,4, Gong-Jian Tang^{1, 2}, Xiu-Zheng Zhang^{1, 2}, William M. White⁵, Qiang Wang^{1, 2, 6}

¹ State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China

² CAS Center for Excellence in Deep Earth Science, Guangzhou, 510640, China

³ Department of Earth Sciences, St Francis Xavier University, PO Box 5000, Antigonish, Nova Scotia, B2G 2W5 Canada

⁴ Earth Dynamics Research Group, The Institute for Geoscience Research (TIGeR), School of Earth and Planetary Sciences, Curtin University, WA 6845, Australia

⁵ Department of Earth and Atmospheric Sciences, Cornell University, Ithaca, New York 14853, USA

⁶ College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing 10049, China

Email: danwei@gig.ac.cn

Extracted and modified from: Dan, W., Murphy, J.B., Tang, G.J., Zhang, X.Z., White, W.M. and Wang, Q. 2022. Cambrian–Ordovician magmatic flare-up in NE Gondwana: A silicic large igneous province? Geological Society of America Bulletin, https://doi.org/10.1130/B36331.1.

Introduction

Mainland northeastern Gondwana and adjacent peri-Gondwanan terranes were affected by regionally widespread Cambrian-Ordovician tectono-magmatic events which include the emplacement of the ~511 Ma Kalkarindji large igneous province (LIP) in northern Australia (Glass and Phillips, 2006; Ware et al., 2018), the opening of the Proto-Qiangtang Ocean at ~500 Ma (Dan et al., 2022), widely distributed Cambrian-Ordovician igneous rocks (Gao et al., 2019; Liu et al., 2019), and a regional unconformity between Cambrian and Ordovician strata (Myrow et al., 2010; Liu et al., 2019). However, the tectonic setting of these Cambrian-Ordovician events, i.e., subduction-related orogenesis (Cawood et al., 2007) versus an extensional setting (Liu et al., 2019) is highly debated. In this contribution, we draw attention to abundant Cambrian-Ordovician igneous rocks in eastern Asia and adjacent regions, which were located south of the Proto-Tethys Ocean and pre-date the Silurian opening of the Paleo-Tethys Ocean (Fig. 1), and summarize the evidence that they may constitute a silicic large igneous province (SLIP).

Definition of the Pinghe SLIP

Duration and pulses of Magmatism

A compilation of zircon U-Pb crystallization ages of Cambrian-Ordovician igneous rocks in NE Gondwana and adjacent peri-Gondwanan terranes is presented in Fig. 2. More than 200 zircon U-Pb ages (including LA-ICPMS and SIMS/SHRIMP methods) loosely constrain their crystallization ages mostly between ~520-450 Ma but with no obvious age peaks (Fig. 2i). The SIMS/SHRIMP zircon U-Pb age data, suitable for deducing the duration of a LIP ((Dan et al., 2021), more tightly constrain the crystallization ages of Cambrian-Ordovician igneous rocks to mostly between 510-460 Ma (Fig. 2j). We maintain that if the age peak is obtained from more than two terranes, this event would be widespread and can be interpreted as a major pulse. Figs. 2j-k show there are two major peaks at ~500 Ma and ~480 Ma, which are prevalent in almost each terrane, and a minor peak at ~466 Ma. We conclude the Cambrian-Ordovician magmatism was generated during 510-460 Ma, but with two major pulses of ~500 Ma and ~480 Ma.

Size and tectonic setting of magmatism

The original distribution of the putative Pinghe SLIP has been disrupted by subsequent tectonic activity along the northern Gondwanan margin and by the >600 km of Cenozoic crustal shorting in the central Himalaya. After accounting for the effects of opening the Paleo-Tethys and Neo-Tethys oceans between peri-Gondwanan terranes, the Cambrian positions of most terranes in NE Gondwana can be reliably located (Fig. 3). The space between the Sibumasu terrane and South China in the paleogeographic reconstruction implies that there may be one unidentified terrane occupying the space between South China and Sibumasu terrane.

Our Cambrian paleogeographic reconstruction implies these igneous rocks were likely originally distributed in an approximately triangular region, >5000 km in length and 1000 km (greatest width), equivalent to an area of ~2.5 Mkm², which far exceeds the minimum size (>0.1 Mkm²) criterion of a LIP. Although a precise estimate cannot be obtained from the available data, their total volume is probably huge (>2.5 Mkm³), if the average thickness of igneous rocks is ~1 km for the enclosed area (Ernst, 2014). Moreover, the Cambrian-Ordovician magmatism was far from the subduction zone, confined to south of the Proto-Qiangtang Ocean, which was opened at ~500 Ma as a backarc basin (Dan et al., 2022).

Composition

The Cambrian-Ordovician igneous rocks are predominantly silicic with minor volumes of mafic rocks (Fig. 1). The mafic rocks are enriched in light rare earth elements (LREE), depleted in high field strength elements (HFSE: Nb, Ta and Ti), and have ε_Nd(t) values of -4.6 to +1.0, implying derivation from a lithospheric mantle previously metasomatized by components derived from a subducted slab (Miller et al., 2001; Liu et al., 2020). In contrast, the silicic igneous rocks are mostly muscovite-bearing peraluminous granites with low ε_Nd(t) values of -11.7 to -4.9 and high zircon δ¹⁸O values of +8.1 ~ +11.4‰, suggesting they were originated by partial melting of metasedimentary rocks (Gao et al., 2019; Dan et al., 2020).

Therefore, the Cambrian-Ordovician magmatic province in NE Gondwana and peri-Gondwana terranes satisfies the criteria of defining a LIP or SLIP, here termed Pinghe SLIP. It is noteworthy that the silicic igneous rocks in this SLIP are S-type granites, in contrast to other SLIPs which are predominantly I-type or A-type igneous rocks (Bryan, 2007; Bryan and Ferrari, 2013; Ernst, 2014), suggesting the composition of silicic igneous rocks may be a less distinguishing characteristic of a SLIP than other criteria (Bryan and Ferrari, 2013).

Geodynamics of the Pinghe SLIP

We propose two possible mechanisms of asthenospheric upwelling herein (Fig. 4). One possibility is that asthenospheric upwelling was induced by a subducted slab that stagnated at mantle transition zone and underwent dehydration causing melting of the upper mantle. This model has been invoked to explain intraplate magmatism that is spatially and temporally connected to a subduction zone but is located several hundred kilometers from a continental arc and is not associated with mantle wedge melting. The model may accommodate the generation of intraplate basalts over a large area (although the volume is minor) far from a subduction zone and includes the possibility of coeval backarc extension adjacent to subduction zone. However, the mafic rocks in the Pinghe SLIP were derived from melting of metasomatized lithospheric mantle (Miller et al., 2001; Liu et al., 2020), different from oceanic island basalts (OIB) occur in this model (Xu et al., 2018). Moreover, this model does not provide a simple explanation for the large-scale melting of crust. Thus, although we don’t prefer this model, it cannot be totally excluded.

Given its regional extent, the origin of Pinghe SLIP may need to be viewed in a global context. The putative Pinghe SLIP occurred after the global-scale Pan-African orogenesis, thus, magmatism of this scale may be a regional response to reorganization of deep mantle convection patterns caused by the subduction of oceanic lithosphere located in a region of downwelling between the converging continents of Gondwana (Heron et al., 2021; Murphy et al., 2021). Numerical models show this region of mantle downwelling should evolve into an upwelling, which would trigger the ascent of deep mantle plumes preferentially along Gondwana’s periphery after its assembly, as reflected in a series of circum-Gondwana plume-related magmatism in that time interval (summarized in Murphy et al. (2021)). Emplacement of a mantle plume beneath the lithosphere would explain several of the observations noted above. It would have resulted in uplift, explaining the regional Cambrian-Ordovician unconformity (Hughes et al., 2011) and ~485 Ma retrograde metamorphism in Lhasa terrane (Zhang et al., 2012). Lithospheric uplift and regional extension would have induced decompression partial melting of the lithospheric mantle, enhanced by heat provided from rising melts from the plume itself, to generate the widespread intraplate continental mafic rocks (Miller et al., 2001). Heat advected upward by these melts would in turn induce partial melting of the thickened Neoproterozoic metasedimentary rocks between Northern Qiangtang-South China and India-Australia to generate the S-type granites that dominate the Pinghe SLIP. Lateral outflow of the plume at the base of the lithosphere would induce rifting, which could have initiated opening of ocean basins (Morgan et al., 2020). A young plume arriving beneath the region provides an explanation for the nearly simultaneous opening of the Proto-Qiangtang oceanic basin (Fig. 4) and initiation of rifting of the Northern Qiangtang and South China terranes from northeast Gondwana, which are coeval with occurrence of the Pinghe SLIP.

The proposed model would be consistent with a plume origin for the adjacent ca. 511 Ma Kalkarindji LIP (Fig. 3) as proposed by Glass and Phillips (2006). Although other mechanisms may have generated the Kalkarindji LIP (Ware et al., 2018), the plume model provides a unified explanation for the coeval occurrence of the SLIP in peri-Gondwanan terranes and adjacent mafic LIP in the Australian part of Gondwana. The correlation of Pinghe SLIP with Kalkarindji LIP is analogous to the well-documented example of ~188-153 Ma Chon Aike SLIP associated with the ~183 Ma Karoo-Ferrar LIP in South America-Antarctic.

Conclusions and perspectives

An extensive belt of ~510-460 Ma granitic rocks, intruded in several pulses over a >2.5 Mkm² area, has been recognized in the northeastern Gondwana. The granitic rocks are mostly of S-type compositions, and were intruded far from (>1000 km) from active convergent margins. This granitic province, which we infer to represent a silicic LIP (Pinghe SLIP), can be explained by the rise of a mantle plume beneath the region. The new identified Pinghe SLIP will motivate to study the link between magmatism and geologically-sudden climate-perturbations, such as extinction at the early-middle Cambrian Boundary and Top Of Cambrian Excursion (Marshall et al., 2021).

Acknowledgement

The authors greatly appreciate Prof. Ernst for encouragement and the invitation to write this article. This study was jointly supported by the National Natural Science Foundation of China (Nos. 92155306, 41872065, 91855215 and 41630208) and the Second Tibetan Plateau Scientific Expedition and Research program (STEP) (Grant No. 2019QZKK0702). JBM acknowledges the continuing funding from NSERC, Canada.

References