Geochemical evolution of basaltic rocks from the Changbai mountains: implications for the nature of lithospheric mantle beneath the ne margin of the Sino-Korean craton

Ming Zhang and Suzanne Y. O'Reilly,

GEMOC, Macquarie

The Late Cenozoic Changbai Mountains (CB) volcanic province is located at the northeastern margin of the Sino-Korean Craton with lava flows distributed in a region of more than 15,000 km2 (Fig. 1). The volcanic activity at the Chinese side of the province can be divided into 6 episodes (Liu, 1988): Zengfengshan (ZF, 20.6-19.8 Ma), West Naitoushan (WNT, 16.7-15.1 Ma), Pingdingcun (PD, 3.0 Ma), Junjianshan (JJ, 2.8 Ma), Guangping (GP, 1.5±0.1 Ma), and Baitoushan (BT, 0.6 Ma - 1702 AD). Products of the first five episodes are basaltic rocks of effusive eruption with an accumulative thickness of >1200 m. Products of the last episode are trachytes to comendites that build up the composite volcano - Baitoushan on the basalt plateau, at the China - North Korean boundary (2691 m above sea level) with a crater lake - Tianchi Lake of ca 8 km2 on the top. The thickness of exposed felsic volcanic rocks is > 900 m. Scattered outcrops of the basement rocks include Archaean granulites and migmatites of the Anshan Group, Mesoproterozoic gneisses, schists and phyllites of the Liaohe Group, and Neoproterozoic (Sinian) to Phanerozoic sedimentary covers. Late Paleozoic and Jurassic (Yanshanian) granites are also intruded in the region. The onset of the CB volcanism is temporally and spatially related to the opening of the Japan Sea and the locus of the Circum-Pacific seismic zone is presently about 550 km beneath the CB province. Therefore, the study of chemical evolution of the CB volcanic rocks will enable us to evaluate not only the nature and evolution of the potential sublithospheric continental mantle (SCLM) source, but also the possible role of the (Paleo-) Pacific Plate subduction in the generation of basaltic magmatism in East China and East Asia.

Petrology and Geochemistry

Fifty-nine samples of volcanic rocks selected from all six episodes have been analysed for major and 34 trace elements using XRF and ICP-MS facilities and subset of 25 samples for Sr-Nd isotope ratios. The representative analytical results are presented in Table 1. The mafic rocks (SiO2=48.85-55.3 wt%) of the first five episodes include alkali olivine basalt (AOB), hawaiite (Haw), mugearite (Mug), olivine tholeiite (OlTh) and quartz tholeiite (QTh), whereas the felsic rocks (SiO2=64.5-77.6 wt%) of the last episode include mainly trachyte (Tra), comenditic trachyte and comendite (Com). Only the basaltic rocks will be discussed in this paper.

MgO contents of the CB basaltic rocks range 10.4-1.4 wt% (corresponding Mg# of 0.72-0.25), accompanied by wide variations in SiO2 and alkaline element contents (Na2O+ K2O=3.9-9.3 wt%). The CB basalts can be broadly divided into an early (21-15 Ma) SiO2-undersaturated alkaline series and a late (< 4 Ma) SiO2-saturated tholeiite series. The oldest Zengfengshan basalts vary widely from slightly evolved ol-tholeiite to highly evolved mugearite, with SiO2 and MgO ranging 48.4-54.8 wt% and 6.6-1.4 wt% (Mg# 0.57-0.25), respectively. West Naitoushan basalts are AOBs and hawaiites with high MgO (10.4-9.7 wt %; Mg# of 0.72-0.71). They are the only CB basalts containing both mantle peridotite xenoliths and high-pressure augite and bronzite megacrysts. In contrast, the younger basalts from Pingdingcun, Junjianshan and Guangping are mostly ol-tholeiites and q-tholeiites with moderately low MgO (6.2-3.9 wt%; Mg# 0.57-0.46). The Pingdingcun basalts and all but one of the Guangping basalts are q-tholeiites, whereas the Junjianshan basalts consist of ³ 11 interbedded lava flows of ol-tholeiite and q-tholeiite. The latest Guangping q-tholeiites are chemically homogeneous with high Al2O3 contents (16.7-17.4 wt% vs 14.8-16.7 wt% for the other tholeiites) and can be petrographically distinguished from the other tholeiites by the presence of abundant (ca 25 vol.%) large labradorite phenocrysts (up to 3 cm) and of both augite and pigeonite as micro-phenocrystic phases.

Incompatible trace element characteristics of the CB basalts can be illustrated using primitive-mantle normalised incompatible element patterns (Fig. 2). The early CB alkaline basalts display broadly similar patterns: they are mostly enriched in Rb (or Ba for evolved Zengfengshan samples) and the degree of enrichment gradually decreases with decreasing incompatibility with several samples having a noticeable trough at Th-U and peaks at K and/or Sr (except for one primitive Zengfengshan basalt). The same pattern has been observed from the latest nephelinites and basanites (70-5 Ka) in the Jingbo Lake province, located at the southern margin of the Xing'an-Mongolian Foldbelt, ca 150 km to the northwest of the CB province (Liu et al., 1994), as well as some oceanic island basalts (eg Gough Island). Heavy rare earth element (HREE) concentrations for the West Naitoushan basalts are low (0.5-1.4 ppm), resulting in the highest normalised La/Yb ratios (11.4-25.7) among the CB basalts. The late tholeiite series basalts, in contrast, show highly fractionated incompatible element patterns: remarkable depletions of Rb relative to Ba, Th, U and Nb to K, La and Ce to Sr and moderate depletion of Zr and Hf relative to Sm. These patterns are identical to the early Tertiary q-tholeiites (45-42 Ma) and Quaternary ol-tholeiites (1.4-0.6 Ma) from the Jingbo Lake (Liu et al., 1994) and resemble those for the Chinese potassic rocks (16.5 Ma-1721 AD, Zhang et al., 1997). The late CB tholeiites have low normalised La/Yb (eg 4.7-8.8 for Pingdingcun q-tholeiites) and ubiquitous positive Eu-anomalies that become more prominent with positive Ba and K anomalies as shown by the Junjianshan tholeiites. The West Naitoushan basalts have primitive mantle Nb/U ratios of 29-33, whereas Nb/U ratios for all the other CB basalts (37-56, except two from Junjianshan) fall well within the range for oceanic basalts (47±10, Hofmann et al., 1986). On the other hand, Ce/Pb ratios for the CB basalts (except two from Guangping) are generally lower than those for oceanic basalts (25±5, Hofmann et al., 1986). A wide range of Ba/Nb (12-120) is accompanied by a limited variation of La/Nb (0.65-1.3) for the CB basalts.

The early CB alkaline basalts and the late CB tholeiites can also be distinguished by their Sr-Nd isotope ratios (Fig. 3). The former are relatively depleted, having low 87Sr/86Sr (0.70436-0.70472) and high 143Nd/144Nd (eNd=+1.2 - +2.4) than the latter (0.70475-0.70514 and +0.3 - -2.4, respectively). Despite the wide variations in elemental chemistry and the relevant parent/daughter element ratios of the CB basalts (eg 87Rb/86Sr=0.025-0.31 for the alkaline series basalts), particularly the Zengfengshan ones, Nd isotope ratios are spectacularly homogeneous for the early alkaline basalts and, to a lesser extent, for the late tholeiites. The alkaline basalts plot in the enriched part of the field for the Hannuoba basalts (Song et al., 1990), whereas the tholeiites plot between the trend for the Chinese potassic rocks (EM1-type, Zhang et al., 1997) and that for the basalts from Fujian and Taiwan, SE China (EM2- type, Chung et al., 1994). Published results (Peng et al., 1986; Basu et al., 1991) and our preliminary Pb isotope data indicate that all the CB basalts bear the Dupal signature (D8/6=+47 - +96) as shown by the Indian Ocean MORB. All but one analysed sample also have high D7/6 (+6.1 - +13.2). Plotting on the right side of the Geochron, the primitive West Naitoushan alkaline basalts are higher in 206Pb/204Pb (18.04-18.08) than the Hannuoba basalts (<18.00, Song et al., 1990) and the other CB basalts (17.46-17.83), which plot on the left side of the Geochron and trend toward the EM1-type Chinese potassic rocks (Zhang et al., 1997).

Petrogenetic Implications

The non-primitive nature of the CB tholeiites and the Zengfengshan alkaline basalts requires evaluation of the role played by assimilation and fractional crystallisation (AFC) processes. Some variations in incompatible element abundances and ratios can be partially attributed to fractional crystallisation, such as the negative correlations between MgO and many incompatible elements for the Zengfengshan basalts. However, this can't account for the diversified incompatible element patterns, including the intersecting REE patterns, displayed by the primitive alkaline basalts and the evolved tholeiites (Fig. 2). Nor can the increase in Ba by a factor of five (550 - 2730 ppm) for the Junjianshan tholeiites can be explained this way as the variations in many other elements for these basalts are so limited (eg MgO 5.0 to 4.2, Ni 100 to 60 ppm, Nb, Sr and Zr virtually constant or even decreased) that only insignificant amounts of fractionation of mafic phases are permitted. Several chemical indicators such as the negative correlation between MgO and 87Sr/86Sr and the positive correlation between MgO and eNd for the Pingdingcun basalts might testify to the potential crustal contamination. However, we suggest that the fundamental differences in geochemical signatures between the two series of CB basalts, such as Sr-Nd-Pb isotope ratios, incompatible element and REE patterns, Ba/Nb, Nb/U and, to a lesser extent, K/Nb, Nb/Eu ratios etc., should be attributed to diversities in mantle sources and/or partial melting processes.

We propose at least two mantle source reservoirs for the CB basalts: an SCLM which made significant contributions to the magma generation, and a sublithospheric mantle (being an Indian-type asthenosphere or, less likely, a mantle plume). Geochemical signatures of the primary West Naitoushan basalts can be explained as results of intensive interactions between the two source components. Temporal chemical variations in the CB basalts manifest a gradual increase in further trapping the enriched shallow SCLM source. The SCLM source could be basically of EM1-type, similar to that which sourced the Chinese potassic basalts although EM2-type geochemical signatures (eg high 87Sr/86Sr) may have been added to the SCLM via either accretion processes at the craton margin since Mesoproterozoic or subduction processes of the Pacific plate before the opening of the Japan Sea. It should be noticed that the old EM1-type SCLM is an essential source component for many basaltic rocks in NE China such as those from Changbai Mts, Jingbo Lake, and Hannuoba and the Chinese potassic rocks. This SCLM is distributed over a large area beneath both the Phanerozoic Xing'an-Mongolian Foldbelt and the northern and northeastern margins of the Sino-Korean Craton in NE China.

References

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Chung, S-L., Sun, S-S., Tu, K., Chen, C-H., and Lee, C-Y., 1994, Late Cenozoic basaltic volcanism around the Taiwan Strait, SE China: product of lithosphere - asthenosphere interaction during continental extension: Chemical Geology, v. 112, p. 1-20.

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Major elements recalculated to 100%; Mg# calculated assuming Fe3+/(Fe3++Fe2+)=0.2.

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