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Petrochemical Study of the Jingpohu Holocene Alkali Basaltic Rocks, Northeastern China

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Petrochemical Study of the Jingpohu Holocene Alkali Basaltic Rocks, Northeastern China Geochemical Journal, Vol. 36, pp. 133 to 153, 2002 Petrochemical study of the Jingpohu Holocene alkali basaltic rocks, northeastern China ZHAOCHONG ZHANG,1* CHENGYOU FENG,2 ZHAONAI LI,1 SHUCAI LI,3 YING XIN,3 ZHAOMU LI3 and XIANZHENG WANG3 1Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China 2Institute of Mineral Deposits, Chinese Academy of Geological Sciences, Beijing 100037, China 3Institute of Geological Sciences of Heilongjiang Province, Ha-erbin 150036, China (Received March 12, 2001; Accepted January 7, 2002) Holocene volcanoes in the Jingpohu region are situated in the “Crater Forest” and “Frog Pool” areas along the northwest side of Jingpohu Lake, northeastern China. Dating of three charcoal samples from the first and second volcanic cycles shows that the ages of the first and second cycles are 3430~3490 and 2470 years respectively. The lavas from the Jingpohu area consist of basanites (BSN), alkali olivine basalts (AOB) and tephrites (TP). Crystal fractionation models are consistent with the generation of AOB and TP from a basanitic parent. Minor fractionation of olivine, augite, magnetite and Cr-spinel is required to produce AOB compositions whereas the generation of TP requires extensive fractionation of kaersutite, phlogopite and anorthoclase with minor olivine, augite, magnetite and leucite. The presence of kaersutite, phlogopite and anorthoclase megacrysts and mantle xenoliths suggest a fractionation history occurring at high pressure in the mantle. Although all basaltic rocks contain many granitic xenoliths, their geochemical characteristics show that they have not undergone any contamination of upper crust en route to the sur- face, but some alkali basalts were suffered from the contamination of lower crust. Relatively unradiogenic isotope ratio (compared with Bulk Earth), steep chondrite-normalized REE patterns and strong incompat- ible element enriched patterns suggest that the magmas are derived from a mixture of an incompatible element depleted anhydrous lherzolite asthenospheric mantle source and an enriched, amphibole-phologite- (apatite-)bearing lherzolite continental lithospheric mantle source. We propose that the basanites are the products of very low degree partial melts (<1%) of this source under high extension strength. processes (e.g., Hofmann, 1988). On the other INTRODUCTION hand, there is increasing evidence that melts origi- The role of the lithospheric mantle in the nating from the asthenosphere interact with the petrogenesis of continental alkali basalts is diffi- lithospheric mantle en route to the surface (Chazot cult to evaluate, as its nature and composition are et al., 1996; Wulf-Pedersen et al., 1996; Class and known to be extremely variable, not easily distin- Goldstein, 1997). Current debate concerns whether guishable from the asthenospheric mantle. Conti- continental basalts are primarily derived from the nental alkali basalts have distinct geochemical sig- asthenospheric mantle, and contaminated by natures compared with mid-ocean ridge basalts lithospheric mantle or if they originate from vari- (MORB), but exhibit many similarities to oceanic ably “metasomatized” lithospheric mantle (e.g., island basalts (OIB). Lithospheric contamination Menzies and Hawkesworth, 1987; Beccaluva et of the asthenospheric mantle may occur through al., 1991; Fancis and Ludden, 1995; Wulf- lithospheric delamination (e.g., McKenzie and Pedersen et al., 1996; Comin-Chiaramenti et al., O’Nions, 1995) or through ancient subduction 1997; Simonetti et al., 1998; Marzoli et al., 2000). *Corresponding author (e-mail: [email protected]) 133 134 Z. Zhang et al. gion. Volcanism occurred frequently from Eocene to Holocene. The Holocene volcanic rocks are dis- tributed in two districts named “Crater Forest” and “Frog Pool” (Fig. 2). The major rock types include alkali olivine basalts (AOB), basanites (BSN) and tephrites (TP). AOB and BSN occur at “Crater Forest”, whereas TP occurs in “Frog Pool”. Based on interlayed sediments, three cycles can be iden- tified. All three cycles are found at “Crater For- est” and two cycles are found at “Frog Pool”. Although a lot of geochemical data for Terti- ary to Pleistocene basalts in the eastern sector of the Jingpohu region have been reported (e.g., Peng et al., 1986; Zhou and Zhu, 1992; Tu et al., 1992; Chen and Xu, 1992; Hsu and Chen, 1998), Holocene basalts discussed here have not been studied in detail. Here we present new 14C, major and trace element, and Sr, Nd and Pb isotopic data, and discuss the implications of those data in terms of petrogenesis. CENOZOIC MAGMATISM IN JINGPOHU Cenozoic basalts in Jingpohu are located at the west side of the Tangcheng-Lujiang (Tan-Lu) fault, which extends for ~2400 km in China and Fig. 1. Map of East China showing the main tectonic continues northeastern into the Russia (Fig. 1), and domains, distribution of Cenozoic volcanic rocks (modi- fied from Fan and Hooper (1991)), and location of Fig. left-laterally horizontally displaces several hun- 2 (square). Abbreviation: XM = Xing’an-Mongolian dred kilometers. The Jingpohu Cenozoic basalts fold belt; NC = North China (Sino-Korean) craton; cover an area of ~15000 km2 with a total volume QL = Qinling fold belt; YZ = Yangtze craton; SC = of >50 km3 and extending nearly 350 km north- South China fold belt. south as a part of Cenozoic belt of eastern China. The magmatism occurred in six episodes, at Eocene (44.9~42.1 Ma), Middle Miocene (16.49~8.65 Ma), Early Pliocene (4.09~3.36 Ma), An opportunity to determine the contribution Late Pliocene (2.6~2.4 Ma), Pleistocene (1.17~0.9 of lithospheric mantle to the petrogenesis of con- Ma) and Holocene (3490~2470 years). Cenozoic tinental alkali basalts is presented by the Jingpohu magmatic activity commenced in the Eocene with Holocene volcanoes, which are formed in eruption of tholeiites, forming two small Heilongjiang Province of northeastern China. The subhorizontally bedded sheets, orientated along a Jingpohu volcanic rocks constitute part of the NE-SW trend. A small scale of eruption along a Cenozoic belt of eastern China (Fig. 1), which is NE-SW trend occurred in Middle Miocene, com- an important component of the circum-Pacific posed of several low lava platform. The Early volcanic belt. The volcanic sequences were dis- Pliocene episode is the major magmatic event in tributed in northeast-southwest trending basins the region, with eruption of alkali olivine basalts, that parallel major tectonic structures in the re- some of which have minor iddingsite rims on The Jingpohu Holocene alkali basaltic rocks, northeastern China 135 Fig. 2. Simplified geological map of the Jingpohu region showing the exposure of Cenozoic volcanic rocks in the β region (modified from Heilongjiang Bureau of Geology and Mineral Resources, 1991). Q4 - Holocene basalt; β 2 β 1 β β Q3 - Pleistocene basalt; Q2 - Late Pliocene basalt; N2 - Early Pliocene basalt; N1 - Middle Miocene basalt; βE - Eocene basalt. Note: some small craters are not shown for clarity. olivine, forming high lava platform in an area of and extends discontinuously 40 km) were formed ~1000 km2. Late Pliocene lavas accumulated within the lava flows. above valley, forming the third terrace. The Except Eocene basalts that belong to tholeeites, Pleistocene magmatic activity formed several those of all other episodes are sodic alkali basalts. small volcanic cones, including basaltic breccias However, compared with the alkali basalts of the intercalated with lavas. These cones also trend a other episodes, the Holocene alkali basalts are NE-SW direction. Holocene basalts are located at richer in alkali. Based on their geochemical and the west side of the Jingpohu volcanic region (Fig. Sr, Nd and Pb data, Liu et al. (1989) suggested 2). They are controlled by NE- and nearly W-E- that tholeiites formed by partial melting at a depth striking structures overlapped by nearly N-S-strik- of ~35–50 km, within the base of the plagioclase- ing structures, and distributed over large areas of lherzolite and the top of the spinel-lherzolite zone the Hercynian-Yanshanian (Mesozoic) granite ter- of the upper mantle whereas the alkali basalts ap- rain. Thirteen composite craters have been recog- pear to have formed at a depth of ~50–60 km en- nized. The craters consist dominantly of thin lay- tirely within the spinel-lherzolite zone of the up- ers of basaltic lavas alternating with volcanic per mantle. bombs, cakes, and scorias. The last stage of lava flows flowed for more than 60 km eastwards along ANALYTICAL METHODS a river valley and blocked the river, thus forming some barrier lakes, such as the well-known Analyses of major elements, REE and other Jingpohu Lake and Xiaobeihu Lake. Long lava trace elements were obtained from Institute of channels (about 1~3 m in width, 0.5~3 m in height Rock and Mineral Analyses, Chinese Academy of 136 Z. Zhang et al. Table 1. 14C isotopic measurement for the Holocene volcanic rocks from the Jingpohu area Sample No. Location Sample Age (B.P.) Correction (A.D.) P33C1451 No. I crater charcoal 4630 ± 60 3430 ± 80 DG-8 No. V crater charcoal 3950 ± 70 2470 ± 120 DG-8 No. V crater charcoal 3970 ± 70 2470 ± 110 TC31 Frog Pool charcoal 4660 ± 80 3490 ± 140 Note: the used half life of 14C is 5568 years; initial age is 1950 years; liquid scintillation meter is Quantulus-1220; all the samples were determined by Yin Jinhui in Earthquake Bureau of China. Geological Sciences (CAGS). Analyses of Sr, Nd Earthquake Bureau of China, Beijing. The method as well as Pb isotopic compositions were carried has been detailed by Stuiver and Reimer (1993), out at the Institute of Geology, Chinese Academy and results are given in Table 1. of Sciences. The analyses of major elements were performed by XRF on a Philips PW2400 auto- PETROGRAPHY mated spectrometer. The REE plus Rb, Sr, Y, Sc, Cr, Ni, Ba, Th, U, Zr, Hf, Nb, Ta were analyzed In this section, all the basaltic samples are by inductively coupled plasma-mass spectrometry fresh, do not contain any secondary minerals, and (ICP-MS, Perkin-Elmer-Sciex Elan Model 500) are mostly strongly porphyritic.
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