The age and geochemistry of the mid-Cretaceous volcanic rocks in the Jinan Basin: Implications for the mid-Cretaceous tectonic environments of the Korean Peninsula and Northeast Asia

Journal Pre-proof The age and geochemistry of the mid-Cretaceous volcanic rocks in the Jinan Basin: Implications for the mid-Cretaceous tectonic environments of the Korean Peninsula and Northeast Asia

Lee Seung Hwan, Oh. Chang Whan, Park Jung Woo PII:

S0024-4937(20)30020-7

DOI:

https://doi.org/10.1016/j.lithos.2020.105383

Reference:

LITHOS 105383

To appear in:

LITHOS

Received date:

15 October 2019

Revised date:

17 December 2019

Accepted date:

15 January 2020

Please cite this article as: L.S. Hwan, O.C. Whan and P.J. Woo, The age and geochemistry of the mid-Cretaceous volcanic rocks in the Jinan Basin: Implications for the midCretaceous tectonic environments of the Korean Peninsula and Northeast Asia, LITHOS(2020), https://doi.org/10.1016/j.lithos.2020.105383

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© 2020 Published by Elsevier.

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The age and geochemistry of the mid-Cretaceous volcanic rocks in the Jinan Basin: Implications for the mid-Cretaceous tectonic environments of the Korean Peninsula and Northeast Asia

Lee Seung Hwana , Oh Chang Whana *, Park Jung Woob a

Department of Earth and Environmental Sciences and The Earth and Environmental

School of Earth and Environmental Sciences, Seoul National University, Seoul 08826,

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Science System Research Center, Chonbuk National University, 54896, Republic of Korea

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Republic of Korea

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*Corresponding author; e-mail address: [email protected]

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Abstract Most of the mid to late Creatacous igneous rocks in the southeastern margin of South China Craton are interpreted to be formed due to upwelling of asthenospheric mantle caused by the back arc extension. However, it is uncertain whether those in the Korean Peninsula formed in the arc tectonic setting or in the back arc extensional setting. The Jinan Basin is one of the

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Cretaceous pull part basins in the southern Korean Peninsula and the volcanic rocks in it give

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important data which can solve the uncertainty. In the Jinan Basin, sedimentation had started

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at least from 97.7 Ma and continued until 89.5 Ma. The sedimentary sequences were

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intermittently extruded by rhyolitic and andesitic magma 90-89 Ma, and finally intruded by basaltic trachyandesite 84 Ma. The volcanic rocks in the Jinan Basin show geochemical

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features of alkaline to high-K calc-alkaline magma series with strong enrichment in large ion lithophile elements and light rare earth elements relative to high field strength elements. The

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mafic and intermediate volcanic rocks are distinguished from the typical continental arc

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magmas by enrichment in incompatible elements such as Y and Zr with high Zr/Y, suggesting

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that they originated from an enriched source. The basaltic trachyandesites and andesite have Sr isotope ratios (87 Sr/86 Sri) of 0.708769-0.709484 and Nd isotope ratios (143Nd/144Ndi ) of 0.511782-0.511886, which indicate that they formed from mantle which was contaminated with ~25% of crustal component in their petrogenesis. The Pb isotope data ( 206 Pb/204 Pb = 17.572-18.158,

207

Pb/204 Pb = 15.535-15.640 and

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Pb/204 Pb = 38.366-38.921) suggest that

the crustal component should have experienced a prolonged evolution with a low μ value. The Mg# (46-57) and Cr contents (40-180 ppm) in the mafic to intermediate rocks higher than typical crustal melts at a given range of SiO 2 , demonstrate that they evolved from a mantle-derived primary magma. We attribute the crust-like geochemical and isotopic signature of the Jinan mafic and intermediate igneous rocks to partial melting of enriched

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lithospheric mantle metasomatized by subduction components a nd subsequent crustal assimilation during magma ascent and pooling at the mid-crustal depth of ~9-10 km. These data represent that the mid-Cretaceous volcanic rocks in the Jinan Basin formed in the back arc extension environment. The mid-Cretaceous basaltic volcanic rocks in the Yeongdong and Gyeongsang Basins locating in the other part of the southern Korean Peninsula, show similar geochemical character suggesting that mid-Cretaceous igneous activity in the southern

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Korean Peninsula formed due to back arc extension.

Keywords: mid-Cretaceous; volcanic rock ; southern Korean Peninsula; slab rollback ; back arc

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extension

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1. Introduction Most mid-Cretaceous (110-80 Ma) igneous activities in the southeastern margin of the Northeast Asian continent were interpreted to be caused by back arc extension due to a rollback of subduction slab (Li. 2000; He and Xu, 2012; Liu et al., 2012,2014, 2016). Whereas the mid-Cretaceous (110-80 Ma) igneous activity in the southern Korean Peninsula

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has remained under debate causing problem for the interpretation on the mid-Cretaceous igneous activities in the Northeast Asia. Early studies suggested that the mid to late-

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Cretaceous igneous rocks in the southern Korean Peninsula formed at a volcanic arc front

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(Hwang and Kima,b, 1994; Yun et al., 1994, 1997; Kim et al., 1998; Zhang et al., 2012 Sung et al., 1998; Sung and Kim, 2012), whereas Kim et al. (2000) noted that those in the

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Gyeongsang Basins show geochemical characteristics of within-plate and post-collisional

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tectonic settings. Recently, Kim et al. (2016) proposed a new tectonic model for the

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Cretaceous tectonic environments of the Korean Peninsula based on spatial and temporal distribution of the igneous rocks. They suggested that slab rollback occurred by steepening of

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the paleo-Pacific plate during 120-100 Ma, leding to intrusion of I- and A-type granitoids due to upwelling asthenospheric mantle into mantle wedge and partial melting of lower crust, respectively (Kim et al., 2016). They argued that shallowing and re-steepening of the paleoPacific plate during 100-80 Ma resulted in arc magmatism represented by calc-alkaline I-type granitoids. On the other hand, Kwon et al. (2013) suggested that the Gyeongsang Basin was under extensional stress probably due to lithospheric thinning at around 100-90 Ma, which caused subsequent asthenospheric upwelling and partial melting of metasomatized lithospheric

mantle,

forming

high-K

calc-alkaline

basalts

and

alkaline

basaltic

trachyandesites. Cretaceous sedimentary basins are widely distributed in the southern Korean Peninsula.

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The Gyeongsang Basin occurs as the largest one in the southern Korean Peninsula along with other small-scale basins classified as a pull-apart basin (Fig. 1; Kim et al., 1997; Lee, 1999; Ryang and Chough, 1997). The mid-Cretaceous igneous rocks mostly occur in and around the basins, suggesting that they have a close genetic relationship with the basin forming process. The Jinan Basin is one of the representative Cretaceous pull-apart basins in the southern Korean Peninsula with mid-Cretaceous igneous activity (Gwag, 1990; Lee, 1992;

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Lee 1999). Therefore, the volcanic rocks in the Jinan Basin will shed new light on the nature of the igneous activity and tectonic environment of the southern Korean Peninsula during

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mid-Cretaceous time. Although several studies on the Cretaceous Jinan Basin were conducted

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to identify the sedimentation time of the Jinan Basin, the sedimemtation tims is still unclear due to insufficient data. Therefore more clear evidence need to identify the sedimentation age

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of the Jinan Basin.

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In this study, we performed zircon U-Pb age dating by a laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) analysis for sedimentary

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and volcanic rocks to constrain the age of sedimentation and igneous activities in the Jinan

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Basin. The whole-rock major and trace elements and Sr-Nd-Pb isotope geochemistry of the volcanic rocks in the Jinan Basin were determined to investigate their petrogenesis and tectonic setting. We will also discuss the petrogeneisis of mid-Cretaceous volcanic rocks in the southern Korean Peninsula and the regional tectonic evolution of the Northeast Asia during the mid-Cretaceous period combining the result of this and previous studies on the Cretaceous igneous rocks in the southern Korean Peninsula and the southeastern margin of the South China Craton (SCC).

2. General geology

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The Korean Peninsula is composed of the three Precambrian basements that are divided from north to south into the Nangrim, Gyeonggi and Yeongnam Massifs. The Yeongnam Massif separated from the Gyeonggi massif by Phanerozoic Ogcheon belt. The collision of North China Craton (NCC) and SCC formed the Qinling- Dabie-Sulu collision belt during the Permo-Triassic era, and the collision belt was expected to extend into Korean Peninsula ( Zhai and Cong 1996; Oh et al., 2005; Oh and Kusky, 2007). The collision event may have

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been followed by the subduction along the southeastern margin of SCC which extended to the southern margin of the Korean Peninsula (Zhou et al., 2006; Oh and Kusky, 2007).

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The subduction-related arc igneous rocks began from Permo-Triassic time and continued

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until ca. 160 Ma in the southern Korean Peninsula (Kim et al., 2011; Kim et al., 2016). Whereas there was no igneous activity betweem 150-120 Ma in the southern Korean

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Peninsula (Kim et al., 2016; Park et al., 2019). This significant magmtic gap could be

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attributed to the flat subduction of the Paleo pacific plate under the Eurasian plate (Sagong et al., 2005; Kim et al., 2016; Park et al., 2019). The igneous activity began again from 110 Ma

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in the southern Korean Peninsula with roll back of Paleo pacific subducting slab due to the

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steeping angle of subduction (Kim et al., 2016; Park et al., 2019). During this time, leftleteral wrench tectonics occurred in the eastern margin of the Asia, forming Crataceous basins due to local extension with transtension (Kim et al., 1997; Chough and Sohn., 2010; Park et al., 2019). The largest Cretaceous basin is the Gyeongsang Basin in the southeastern part of the Korean Peninsula, and other basins are classified as small-scale pull-apart basins: they are Haenam, Yeongdong, Gongju, Eunsung, Pungam, Muju, Gunsan and Jinan Basins (Fig. 1). These basins are mainly composed of non-marine volcanic sediments and terrigenous sediment and formed along the strike-slip faults with sinistral movement (Ryang and Chough, 1997; Lee, 1999; Chough and Sohn., 2010). Most representative strike-slip fault systems are the Gongju- Eumsung and Yongdong-Kwangju fault systems with the NNE trend

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(Ryang and Chough, 1997; Lee, 1999). The Jinan Basin which is focused on this study, is located within the northwestern Yeongnam Massif and is in contact with the Ogcheon belt. The Jinan Basin was developed along the Yongdong-Kwangju fault system and bounded by two strike-slip faults, the Kwangju and Jeonju faults. The basement rock of the Jinan Basin is Paleoproterozoic granitic gneiss and Jurassic granite (Kim et al., 1984; Lee and Chough, 1999; Shimamura, 1924). The

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Jinan Basin is asymmetrical with a half- graben geometry along the southeastern margin (Baag and Kwon, 1994; Lee and Chough, 1999). It consists of four formations: the

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Sansudong Formation (sandstone, black shale, 600m thick), the Dalgil Formation (tuffs,

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black shale and tuffaceous shale, 1,000m thick), the Manduksan Formation (conglomerate, sandstone, black shale, andesite, tuffaceous shale, tuffs, and mudstone, 800m thick), the

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Maisan Formation (conglomerate, 1500-2000m thick) (Fig. 2). These four formations

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correspond to contemporaneous heteropic facies deposited in laterally different sedimentary environments (Lee, 1992). A plant fossil, acmaea of Frenelopsis sp., found in the Sansudong

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formation indicates that sedimentation occurred between Barremian and Aptian in the Jinan

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Basin (Reedman and Um, 1975; Chang, 1975). These sedime ntary rocks are interlayered or intruded by the Cretaceous basalt, andesite, andesitic tuff, rhyolite, rhyolitic tuff (Shimamura, 1924; Gwag, 1990; Kim et al., 1984; Lee, 1992). Son (1969) correlated the felsic volcanic rocks in the Jinan Basin to the Sakugi Series with Campanian to Maasstrichtian ages whereas Chang (1975) correlated the andesitic rocks to lower Yucheon formation (Albian) in the Gyeongsang Basin.

3. Samples and Petrography The Cretaceous mafic to felsic volcanic rocks and sub- volcanic rocks occur in and around the Jinan Basin. Massive trachybasaltic enclaves occur in andesite with fine- grained matrix in

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the southwestern Jinan Basin (Fig. 3a). The trachybasalt has plagioclase and clinopyroxene phenocrysts within the matrix mainly composed of glass and fine- grained acicula plagioclase (Fig. 4a). Basaltic trachyandesite extruded on top of the Cretaceous quartz porphyry in the southwestern Jinan Basin and intruded the Sansudong formation in the central area of the Jinan Basin. The extrusive basaltic trachyandesite is dark gray in color and fine-grained (Fig. 3b). Clinopyroxene occurs as a phenocryst in the matrix which consists mainly of fine-

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grained plagioclase, glass and cryptocrystalline minerals (Fig. 4b). On the other hand, intrusive basaltic trachyandesite is dark greenish and coarser-grained compared to extrusive

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basaltic trachyandesite (Fig. 3c,c-1). Clinopyroxene and plagioclase are major phenocrysts of

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the intrusive basaltic trachyandesite showing ophitic texture (Fig. 4c). The andesites occur in the southwestern area of the Jinan Basin (Fig. 3d). The phenocrysts are mainly clinopyroxene

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and plagioclase. Some clinopyroxene grains show glomerophyric texture (Fig. 4d). Their

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matrix is composed of glass and fine-grained plagioclase. Rhyolites intruded the Sansudong formation in the central area of the Jinan Basin. They show porphyritic texture with abundant

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quartz and feldspar phenocrysts (Fig. 3e,e-1; Fig. 4e) within a matrix comprised of fine-

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grained biotite, quartz and K-feldspar. Pyroclastic rocks occur in the upper part of volcanic successions. They consist mainly of angular or rounded pyroclasts with various color and size (3~5cm) and fine- grained matrix consisting of quartz and cryptocrystalline minerals. (Figs. 3f, 4f).

4. Analytical method The mineral compositions were measured using a Shimadzu EPMA 1600 instrument at the Korea Basic Science Institute (KBSI), Jeonju, Korea. The operating conditions are as follows: a 15-kV accelerating voltage, a 20 nA beam current and a 1~3 μm probe diameter. The ZAF method was used for matrix correction. The representative mineral compositions from the

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basaltic trachyandesite, andesite and rhyolite are summarized in supplementary Table 1 and 2. The mafic to felsic volcanic rocks in the Jinan Basins were collected for major and trace element analysis. Those samples were crushed to the point at which 90% passed a 10mesh sieve and were split to yield an ~250g sample. The split samples were pulverized to the point atwhich 95% passed a 200mesh sieve to provide a homogeneous and representative sample for analysis. The whole rock chemistries of the igneous rocks in the Jinan Basin were

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analyzed using inductively coupled plasma atomic emission spectrometr y (ICP-AES; Termo Jarrel Ash ENVIRO II) and inductively coupled plasma mass spectrometry (ICP-MS; Perkin

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Elmer Optima 3000) in Activation Laboratories Ltd., Canada. The results are shown in

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supplementary Table 3.

The whole rock isotopic ratios of Sr, Nd and Pb were determined by a VG Sector thermal

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ionization mass spectrometers (TIMS) at the Korean Basic Science Institute (KBSI), Ochang,

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South Korea. Replicate analysis of the NBS987 and JNdi-1 standards gave 143

Sr/86 Sr =

Nd/144 Nd = 0.512110 ± 0.000006 (N=10, 2σ)

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0.710260 ± 0.000004 (N=10, 2σ) and

87

was

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respectively. Pb isotopic ratios were corrected based on replicate analysis for NBS981, which Pb/204 Pb = 16.942 ± 0.005,

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Pb/204 Pb = 15.496 ±0.006 and

208

Pb/204 Pb = 36.701 ±

0.011, (N=16, 2σ). Total procedural blanks level was below 0.1ng for Sr, Nd and Pb. The results are given in supplementary Table 4. For zircon U-Pb age dating in the mafic to felsic volcanic rocks in the Jinan Basin, zircon grains were separated by the standard crushing, water-based panning, magnetic separation and heavy liquid techniques. The separated zircon grains were handpicked and mounted with standard zircons of 91500 (1062.6±1.2 Ma) and Plesovice (337.6±0.69 Ma) in an epoxy disk. The internal textures of the zircon grainswere examined by cathodoluminescence (CL) images obtained by scanning electron microscopy with a JEOL JSM–6610 LV

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instrument.The in-situ zircon ages were analyzed by LA-MC-ICP-MS at the Ohchang center, Korea Basic Science Institute (KBSI), which is composed of a Nu Plasma II MC-ICP-MS and a NWR193UC (193nm) laser ablation system. Common Pb was removed by the

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Pb

correction method using the model by Stacy & Kramers (1975). The zircons U-Pb age data are summarized in supplementary Table 5.

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5. Results 5.1 Mineral chemistry

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Clinopyroxenes occur as phenocrysts in the basaltic trachyandesite and andesite in the Jinan

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Basin. They are mostly diopsidic and have Mg# (Mg# = molar Mg2+/(Mg2++Fe2+) * 100) of 88.2-79.5 for basaltic trachyandesites and 79.3-70.9 for andesites. Plagioclases from the trachyandesite

(JA161015-1,

JA161231-1B)

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basaltic

and

andesite

(JA151010-1A)

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corresponds to albite with 0.90-0.83 XAb (JA161015-1), 0.85-0.35 XAb (JA161231-1B) and 0.95-0.74 XAb respectively. The plagioclase and K-feldspar within rhyolite (JA151231-3F),

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which intruded the Sansudong formation in the central area of the Jinan Basin, have XAb of

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0.98-0.99 and XOr. of 0.98-0.96, respectively. In the rhyolite (JA161201-1B) which extruded in the northern area of the Jinan Basin, XAb ratios of plagioclase are 0.91-0.97. The Kfeldspar within rhyolite in northwestern part belongs to orthoclase with 0.96-0.97 XOr.

5.2 Whole rock chemistry The volcanic rocks in the Jinan Basin have moderate to high losses on ignition (1.3-9.2) indicating possibility of hydrothermal alteration. However, Ce* values of all volcanic rocks show 1.00-1.04 within the immobile range (immobile range: 0.9
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All the volcanic rocks in the Jinan Basin exhibit geochemistry of subalkaline high-K calcalkaline series except for a mafic enclave hosted by the andesitic rock. Those volcanic rocks are plotted in the basaltic trachyandesite (SiO 2 = 52.8-54.5 wt%, Na2 O+K 2O = 4.8-6.0 wt%), andesite (SiO 2 = 57.1-58.2 wt%, Na2 O+K2O = 5.6-5.8 wt%) and rhyolite (SiO 2 = 71.9-75.7 wt%, Na2 O+K 2 O = 6.8-8.4 wt%) field respectively (Figs. 5a, b and c). The enclave mafic volcanic rock is alkaline (Na2 O+K 2O = 7.4 wt.%) and classified as a trachybasalt in the total

Jinan

Basin

are

plotted

in

the

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alkali Na2 O+K2 O vs. SiO 2 diagram (SiO 2 = 50.3 wt %) (Figs. 5a, b). The rhyolites in the peraluminous

field

with

A/CNK

(molar

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Al2 O 3 /CaO+Na2 O+K 2O)) value of 1.11-1.20. (Fig. 5d). Our results are consistent with the

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mid-Cretaceous volcanic rocks in and around the Jinan Basin previously reported by Kim (2015) (Fig. 5).

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The volcanic rocks in the Jinan Basin are plotted on the several tectonic discrimination

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diagrams together with the mid-Cretaceous volcanic rocks in the Yeongdong and Gyeongsang Basins in Figures 6 and 7. The rhyolites in the Jinan Basin are characterized by

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high Rb, Y, and Nb contents relative to typical volcanic arc granites, being plotted in the post

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collision granite field in the Rb vs. Y+Nb discrimination diagram (Fig. 6a). Some of them contain incompatible element contents as high as A-type granites at a given FeO/MgO ratios and are classified as an A2 (post collisional) type in the Nb–Y–Ce discrimination diagram (Figs. 6b, c). The felsic igneous rocks in the Yeongdong and Gyeongsang Basins also show similar geochemical features (Figs. 6b, c). The mafic volcanic rocks in the Jinan Basin are plotted on the back arc basin basalt field in the V-Ti/1000 tectonic discrimination diagram and on the field of within plate basalt in the Zr/Y- Zr and Cr-Y tectonic discrimination diagrams together with mafic volcanic rocks in the Yeongdong and Gyeongsang Basins (Figs. 6 d, e,f). All volcanic rocks from the Jinan Basin show fractionated rare earth element (REE)

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patterns with light rare earth element (LREE) enrichment and heavy rare element (HREE) depletion ((La/Yb)n = 6.86-14.32) and rhyolite show negatice Eu anomalies (Eu/Eu* = 0.150.49) in the CI Chondrite- normalized REE diagram (Figs. 7a,c,e). In the Primitive Mantle normalized diagram, the mafic to intermediate igneous rocks show similar patterns with strong Nb, Ta and Ti negative anomalies and moderate Zr and Hf positive anomalies (Figs. 7b, d). Felsic igneous rocks in the Jinan Basin have the similar patterns of the mafic and

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intermediate rocks, but characterized by marked Ba, Sr and P negative anomalies and more

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pronounced Nb, Ta and Ti negative anomalies (Fig. 7f).

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5.3 Isotope chemistry

Whole rock Sr, Nd and Pb isotopic composition of basaltic trachyandesite and andesite in the

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Jinan Basin are shown in Figures 8 and 9. Initial Sr, Nd and Pb isotope ratios are obtained by

trachyandesites have

Sr/86Sri = 0.708769-0.709012,

Pb/204 Pb = 17.572-17.704,

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143

Nd/144Ndi = 0.511782-0.511836,

Pb/204 Pb = 15.535-15.560 and

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206

87

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assuming 90 Ma for andesite and 84 Ma for basaltic trachyandesites. The basaltic

208

Pb/204 Pb = 38.366-38.428.

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The andesite shows Sr and Nd isotopic compositions (87 Sr/86 Sri = 0.709484,

143

Nd/144Ndi =

0.511886) similar to those of basaltic trachyandesites, although it has slightly higher 206

Pb/204 Pb (= 18.158),

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Pb/204 Pb (= 15.640) and

208

Pb/204 Pb (= 38.921). The Sr and Nd

isotopic ratios of basalts in the Yeongdong Basin (Sagong et al., 2001) are comparable to those of volcanic rocks in the Jinan Basin. The mid-Cretaceous basalts from the Gyeongsang Basin (Kwon et al., 2013) have wider range of 87 Sr/86 Sri and

143

Nd/144 Ndi ratios than those of

volcanic rocks in the Jinan Basin. The basement of the Jinan Basin is composed of the Paleoprotrozoic gneiss and Jurassic granite which have highly enriched isotopic compositions compared to those of volcanic rocks in the Jinan Basin (Fig. 8; Shin et al., 2001; Lee et al., 2005). All mafic and intermediate volcanic rocks in the Jinan, Yeongdong and Gyeongsang

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Basins have Sr and Nd isotopic values between those of the depleted mantle and the basement rocks. The Pb isotopic values of volcanic rocks in the Jinan Basin are lower than those of the Yeongnam Massif and are plotted between those of depleted mantle and the Yeongnam Massif with andesite being more radiogenic in Pb isotope ratios than basaltic trachyandesite (Fig. 9). The basaltic trachyandesite are plotted on the left side of the geochron in the

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Pb/204 Pb-206 Pb/204 Pb diagram suggesting a prolonged evolution in a source material

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with low μ value (Fig. 9b).

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5.4 Pre-eruptive pressure and temperature

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The pre-eruptive conditions can be constrained by clinopyroxene phenocrysts in basaltic trachyandesite and andesite using clinopyroxene- melt geothermobarometer (Putirka et al.,

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2008). In this study, compositions of clinopyroxene phenocrysts and host rocks were used to

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estimate the pre-eruption temperature and pressure with the assumption that the whole rock composition represents the liquid composition, which is reasonable because the basaltic

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trachyandesite and andesite contain < 5 vol.% phenocrysts. The equilibrated pairs of

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clinopyroxene and liquid with KD(Fe-Mg)Cpx-liquid range of 0.20 – 0.36 were used to estimate temperature and pressure, as suggested by Putirka et al. (2008). The pre-eruptive pressure and temperature data are summarized in supplementary Table 6. The pre-eruptive temperature of basaltic trachyandesite ranges from 1101.8℃ to 1117.5℃ with an average of 1109.5℃. The estimated pressure ranges from 2.1 kbar to 3.4 kbar with an average of 2.9 kbar, which is equivalent to a depth of 10.7 km. The pre-eruptive temperature and pressure of andesitic magma are 1087.3℃ to 1102.7℃ with an average of 1094.2℃ and 1.8 ~ 3.1 kbar with an average of 2.6 kbar (equivalent to 9.6 km depth) respectively. These data indicate that the Jinan basaltic trachyandesite and andesite magmas stayed at

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shallow to mid crust of ~ 10 km depth during their ascent through the continental crust.

6. Geochronology To constrain the deposition age of the Jinan Basin, sedimentary rocks from the lowest sequence in the central part and the top layer in the northwestern margin of the Jinan Basin were selected for U-Pb zircon age dating analysis. Total seven samples of basaltic

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trachyandesite, andesite and rhyolite were selected for U-Pb zircon age dating to investigate

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the crystallization ages of the volcanic and sub-volcanic rocks in the Jinan Basin.

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6.1 Zircon U-Pb age dating for sedimentary rocks

The sandstone JA151231-2A was collected from the lowest sedimentary unit in the central

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part of the Jinan Basin. The zircon grains are mostly medium size (100~200μm) with length-

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to-width ratios of 3:1 to 1:1. They commonly exhibit rounded shape with concentric and

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banded zoning patterns although some are prismatic (Fig. 10a). U-Pb isotope data for 78 grains from this sandstone are plotted on a Concordia diagram (Fig. 11a). The concordant U/206 Pb ages range from Paleoproterozoic (2,447±7.3 Ma) to Cretaceous (93.7±1.7 Ma)

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238

with three peaks: Paleoproterozoic, Jurassic and Cretaceous. The weighted mean

238

U/206 Pb

age of the youngest zircons (n = 5, MSWD = 4.8) is 97.7±2.9 Ma, indicating that deposition began at least after ~ 97 Ma. The sandstone JA160116-4A collected from the top seimentary unit in the northwestern margin of the Jinan Basin has zircon grains with small to medium size (100~200μm) whose length-to-width ratios are 3:1 to 1:1. They show prismatic to rounded shape with concentric and banded zoning patterns (Fig. 10b). Among the 80 grains with concordant 238 U/206 Pb ages, most detrital zircons give Cretaceous ages while there are a few detrital zircons with Neoproterozoic (916.2±5.5 Ma) and Jurassic (147.6±2.8 Ma) ages. The weighted mean 238 U/206 Pb

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age of the youngest zircons (n = 7, MSWD = 0.55) is 89.47±0.51 Ma (Fig. 11b) indicating that the sedimentation of top unit in the northwestern margin occurred after 89.47±0.51 Ma.

6.2 Zircon U-Pb age dating for mafic to intermediate igneous rocks The zircons from the andesite JA160102-7C are small to medium-size (100~200μm) with length-to-width ratios of 3:1 to 1:1. Most zircons show rounded shape with banded and sector 238

U/206Pb ages of 90.30±0.17 Ma (n

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zoning patterns (Fig. 10c). The igneous zircons provide = 24, MSWD = 1.9). (Fig. 11c).

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The zircon grains from the basaltic trachyandesite JA151231-1A are angular and prismatic

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and 100~150μm in size with length-to-width ratios of 2:1 to 1:1 (Fig. 10d). 238 U/206 Pb isotope data for 40 grains are plotted on a Concordia diagram (Fig. 11d). The zircons rims give a 238

U/206 Pb age of 84.56±0.45 Ma (n = 31, MSWD = 14). Note that three

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weighted mean

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zircon grains contain inherited cores with Paleoproterozoic (1,901 and 1,987 Ma, n=2) and Jurassic (163 Ma, n=1) ages. This basaltic trachyandesite intruded into the Sansudong and

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Dalgil Formations in the central part of the Jinan Basin.

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The zircons from the andesite JA151015-2A are mostly small- size (50~100μm) with length-to-width ratios of 3:1 to 1:1. They exhibit angular, prismatic or rounded shape with concentric zoning patterns (Fig. 10e). The U-Pb isotope data for 38 grains are plotted on a Concordia diagram (Fig. 11e). The weighted mean of the youngest age is 84.2±1.1 Ma (n = 8, MSWD = 3.3) (Fig. 11e). This age is similar to the intrusion age of basaltic trachyandesite in the central part of Jinan Basin. There are abundant inherited cores whose concordant ages are Paleoproterozoic (2,469-1,832 Ma, n=5) and Jurassic (184-160 Ma, n=5). 6.3 Zircon U-Pb age dating for rhyolite The zircons from the rhyolites JA151231-3F and JA160109-4, which intruded the sedimentary rocks of the Sansudong Formation, are usually small to medium-size grains

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(100~250μm) with length-to-width ratios of 3:1. Some zircons in the rhyolites JA151231-3F have grain sizes up to ~600μm. Most zircons show prismatic shape and well de veloped igneous zoning (Figs. 10f, g). The results of U-Pb isotope analyses on 40 zircon grains from JA151231-3F and 30 grains from JA160109-4, are plotted on a concordia diagram (Figs. 11f, g). The weighted mean 238 U/206 Pb intrusion age of former and latter rhyolites area 89.66±0.37 Ma (n =26, MSWD = 2.3) and 89.25±0.21Ma (n = 27, MSWD = 0.25), respectively (Figs.

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11f, g).

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7. Discussion

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7.1 The evolution of the Jinan Basin

The Jinan Basin has been considered as a pull-apart basin formed during Cretaceous, but

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the time frame of sedimentation and igneous activities have not been clearly determined. The

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beginning of sedimentation in the Jinan Basin was suggested to be roughly 129-113 Ma based on the plant fossil (Reedman and Um, 1975; Chang, 1975). However, this study shows that

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the youngest detrital zircon age of the lowest sedimentary sequence of the Sansudong

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formation is ~ 97.7 Ma. The youngest detrital zircon age of the top sedimentary unit in the northwestern margin is ~89.5 Ma and the youngest volcanic rocks interlayered with sedimentary rocks in the Jinan Basin is ~90 Ma. These ages suggest that the sedimentation may have begun after ca. 97 Ma and continued to 89 Ma. The sedimentary rocks in the Jinan Basin can be correlated with the sedimentation events of the Hayang and Yucheon Formations in the Gyeongsang Basin. This result indicates that the sinistral strike-slip movement along faults on northwestern and southeastern margins of the Jinan Basin may have occurred from ~97 Ma to at least 89 Ma, resulting in the formation of the pull-apart basin with volcanic activity (Lee, 1999; Chang, 1975). During the period, other pull-apart basins in the southern Korean Peninsula may have also formed with volcanic activities.

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7.2 Petrogenesis on the mid-Cretaceous volcanic rocks in the Jinan Basin The mafic and intermediate volcanic rocks in the Jinan Basin show geochemical characteristics of alkaline to high-K calc-alkaline series magmas with strong enrichment in large ion lithophile elements (LILE) and LREE, depletion in HREE and negative anomaly of Nb, Ta and Ti, (Figs. 7a-d). The similar trace element patterns between the mafic and

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intermediate rocks, suggest their comagmatic origin. Although the rhyolites show the trace element geochemistry comparable to those of trachybasalt, basaltic trachyandesite and

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andesite, their trace element patterns exhibit more pronounced Nb, Ta and Ti negative

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anomalies with strong Sr and P negative anomalies (Figs. 7e, f). It can be attributed to extensive fractional crystallization of plagioclase, apatite and Nb-Ta-Ti bearing oxide

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minerals during magma differentiation in the crust.

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The enrichment of fluid- mobile elements with Nb-Ta negative anomalies in the primitive mantle-normalized trace element patterns, is a typical geochemical feature of arc related

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magmas (Figs. 7b, d, f). However, their geochemistry is distinguished from the typical arc

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igneous rocks. For example, the mafic rocks have significantly higher Zr and Y contents with higher Zr/Y ratios than the typical island and continental arc basalts, which are comparable with within plate basalts in the Zr vs Zr/Y and Y vs Cr diagrams (Figs. 6e, f). They also exhibit higher Ti/V ratios than typical island arc tholeiites, suggesting that they originate from more fertile and less oxidized mantle sources than typical sub-arc mantle (Fig. 6d). These geochemical features are similar to those of mid-Cretaceous alkaline basalt and basaltic trachyandesite from the Yeongdong (95-85 Ma; Sagong et al., 2001) and midCretaceous Gyeongsang Basins (ca. 100Ma; Kwon et al., 2013) (Figs. 5, 6), In addition, some of the rhyolites from the Jinan Basin show incompatible element enriched features with higher Zr, Ce, Y and Nb contents at a given FeOt/MgO than felsic

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igneous rocks from the normal volcanic arc systems (Fig. 6b), displaying geochemical features of A2-type granites that formed in post collisional tectonic environment (Fig. 6c). The depletion of Nb and Ta compared to LILEs, together with Sr and Nd isotopic compositions of the mafic volcanic rocks (87 Sr/86 Sri = 0.708769 to 0.709012; εNd(t) = -14.7 to -16.7) represent involvement of old crustal components during their magma genesis (Figs. 7, 8). It may be ascribed to dehydration or water-added partial melting of crustal materials,

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however, such processes are unlikely to explain the mafic nature of the trachybasalt and basaltic trachyandesite. Figure 12a-c compares geochemistry of the Jinan volcanic rocks with

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those of experimental melts produced by dehydration or water-added melting of basaltic

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amphibolite (Lu et al., 2015 and references therein). The trachybasalt, basaltic trachyandesite,

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and andesite have higher Mg#, MgO and Cr contents than volcanic rocks formed by melting

derived from mantle.

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of mafic lower crust at the same range of SiO 2 , suggesting that the Jinan primary magma was

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Kwon et al. (2013) investigated geochemistry of mid-Cretaceous basalt and basaltic

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trachyandesite in the Gyeongsang Basin which is also characterized by the 'crust- like’ primitive mantle- normalized trace element patterns and enriched isotopic geochemical features (87 Sr/86 Sri = 0.70576 to 0.71145; εNdi = 0 to -14) (Fig. 8). They suggested that the mafic magmas originated mainly from lithospheric mantle contaminated by crustal materials such as foundered eclogite or subducted altered oceanic crust and pelagic sediment with varying degrees of subsequent crustal assimilation during ascending through the crust. Sagong et al. (2001) also ascribed the enriched Nd-Sr-Pb isotope signatures of basalts from the Yeongdong Basin to an enriched mantle lithospheric source. The mafic volcanic rocks from the Jinan Basin exhibit geochemical and isotopic similarities with the mid-Cretaceous mafic rocks from the Yeongdong and Gyeongsang Basins, suggesting that the primary

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magma of the Jinan volcanic rocks were also originated from an enriched lithospheric mantle. The enriched isotopic signatures of the basaltic trachyandesite and andesite in the Jinan Basin can be originated from the depleted mantle source which was contaminated with crustal material. However, trace element composition of basaltic trachyandesite and andesite are more enriched than those of average bulk continental crust. If depleted mantle source was contaminated by Paleoproterozoic basement, trace element content of basaltic trachyandesite

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and andesite in the Jinan Basin should be lower than those of bulk continental crust, suggesting the enriched lithospheric mantle origin for them. Figure 12d compares major

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element compositions of the mid-Cretaceous mafic volcanic rocks from the three basins with

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experimental melt compositions produced by phlogopite (± amphibole) peridotite. Their composition is the most comparable with partial melts of spinel-phlogophite peridotite,

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displaying lower CaO/Al2 O3 and higher K 2 O/MgO ratios than partial melts of anhydrous

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peridotite.

However, the presence of inherited zircons in the Jinan basaltic trachyandesite and andesite,

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together with peraluminous nature of rhyolite imply that the Jinan magma underwent crustal

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assimilation to some extent. In an attempt to constrain the source and degree of crustal contamination of the Jinan volcanic rocks, we conducted mass balance models using Sr-Nd isotope and trace element geochemistry of possible contaminants and a primary magma source. For crustal contaminants, the Paleoproterozoic granitic gneiss ( Lee et al., 2005) and the Jurassic granite (Shin et al., 2001) that constitute the basement of the Jinan Basin were chosen. It is consistent with the age distributions of inherited zircon cores from the basaltic trachyandesite and andesite. The least enriched compositions of the early to mid-Cretaceous basalts in the southeastern part of the SCC (Chen and Zhou, 1995; Lapierre et al., 1997; Liao et al., 1999; Zhou and Li, 2000; Zhou et al., 2015) formed by enriched lithospheric mantle melting can be considered as a primary magma composition because these rocks are

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suggested to be produced under the tectonic environment similar to the southern part of Korean peninsula during the mid-Cretaceous (see references in the section 7.3.). The results show that the Sr-Nd isotopic compositions of basaltic trachyandesite and andesite are best explained by mixing between the SCC basalt (Sri=0.704089, Ndi=0.512760) and the Paleoproterozoic granitic gneiss (Sri=0.7476348, Ndi=0.51111786) with a ratio of ~ 3:1 (~ 25% crustal contamination; Fig. 8). The Jurassic granite that occurs around the Jinan

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Basin is less likely to be a prominent contaminant source because it requires an unrealistically large degree (>40 %) of crustal contamination. This result agrees with those of trace element

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models, which show ~10-30 % crustal contamination by the Paleoproterozoic granitic gneiss

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(Fig. 13). The andesite exhibits higher Nb/U and Ce/Pb ratios than basaltic trachyandesite with slightly more radiogenic Pb isotopes, showing progressive crustal assimilation during

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magma evolution from trachyandesite to andesite.

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It should be noted that these models have large uncertainties introduced from the selection of end members and their compositions. For example, isotope and trace element

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geochemistry of primary melts derived from an enriched lithospheric mantle can be highly

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variable and often display strongly enriched characteristics (87 Sr/86 Sri ~0.707660, εNdi ~0.511595, εHfi ~ -10; Menzies and Halliday, 1988; Couzinié et al., 2016). Therefore, the estimated degree of crustal contamination of ~ 25 % for the Jinan volcanic rocks can be lowered if the assumed primary melt is more enriched. However, these models are still meaningful in testing the possible crustal sources because the general mixing trends will not be significantly affected by variations in a primary melt composition unless the enriched lithospheric mantle composition substantially deviates from the mantle array.

7.3 Tectonic implications of the mid-Cretaceous volcanic rocks in the Jinan Basin The Cretaceous igneous rocks in the southern Korean peninsula have been considered to

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form in a typical continental arc tectonic environment in the previous studies (Lee et al., 1987;Hwang and Kim, 1994a,b; Yun et al., 1994, 1997; Kim et al., 1998; Sung et al., 1998; Sung and Kim, 2012; Zhang et al., 2012). Cheong and Jo (2017) conducted comprehensive geochemical studies on late Cretaceous to Paleogene granitoids in the Gyeongsang Basin and proposed that they are products of crustal reworking of the Paleozoic and early Eocene igneous basements.

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Kim et al. (2016) suggested that 120-110 Ma I- and A-type granitoids, which are mainly located in the northwestern part of the Korean peninsula, formed by rollback of the paleo-

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Pacific plate and subsequent asthenospheric upwelling which caused partial melting of

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continental lower crust and strong assimilation of lower crustal materials with mantle-derived basaltic magma. They also argued that intermittent slab bucking might have resulted in

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steepening of the subducting slab between 110 and 100 Ma, leading to backarc extension and

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formation of the Gyeongsang basin as well as the small-sized pull-apart basins in the southern Korean peninsula. They suggested subsequent increasing convergence rate caused typical arc

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magmatism with I-type granitoids began since 100 Ma.

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However, the occurrence of mid-Cretaceous alkaline and high-K calc-alkaline trachybasalt and basaltic trachyandesite from the Jinan Basin (this study) as well as the Gyeongsang and Yeongdong Basins (Sagong et al., 2001; Kwon et al., 2013), which are produced by partial melting of enriched lithospheric mantle, suggests that the southern part of Korean Peninsula was under an extensional regime at least until ~ 85 Ma. Kwon et al. (2013) suggested that oblique subduction of the paleo-Pacific plate during early Cretaceous might have induced initiation or reactivation of NE-striking sinistral faults in the southern parts of Korean Peninsula, resulting lithospheric thinning and upwelling of asthenospheric mantle, which may have resulted in partial melting of enriched lithospheric mantle metasomatized during a previous subduction period.

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We suggest that the back arc extension of the southern part of Korean Peninsula and subsequent enriched lithospheric melting may have resulted from roll-back and steepening of the subducting paleo-Pacific plate as in the southeastern margin of the SCC based on following arguments (Fig. 15). It has been suggested a long subduction system along the margin of Northeast Asia from the southeastern margin of the SCC through southern part of Korea to Japan (Fig. 1; Xu et al., 1987). There was a significant change in tectonic

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environments of the southeastern SCC during 140-80 Ma from a compressional stress regime to an extensional stress regime mainly due to the slab rollback with increasing subduction

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angle (Li. 2000; He and Xu, 2012; Liu et al., 2012, 2014, 2016). It led to back arc extension,

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the upwelling of asthenospheric mantle and partial melting of the lithospheric mantle and crust which were enriched by subduction components during previous subduction events (Li,

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2000; He and Xu, 2012; Liu et al., 2012, 2014, 2016). The mid-Cretaceous igneous rocks

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from the southeastern margin of the SCC exhibit the geochemical features similar to those from the southern parts of Korean peninsula with displaying enriched isotopic features (Figs.

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8, 14), inferring the mid-Cretaceous igneous rocks in both areas may have been produced in a

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similar tectonic environment. It can be suggested that the magma activity related to the back arc extension had started from 140 Ma in the southern part of the eastern margin of the SCC and propagated to the northern part during 120-110 Ma and to the southern Korean Peninsula during 110-80 Ma (Li, 2000; this study). More study will be needed to confirm this suggestion.

Conclusion 1) The youngest detrital zircon ages from the lowest and upper parts of the sedimentary units in the Jinan Basin are 97.7 ± 2.9 Ma and 89.5 ± 0.51 Ma respectively. It indicates that the sedimentation process in the Jinan Basin had started at least from ca. 97.7 Ma and continued

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until ca. 89.5 Ma. 2) Andesite and rhyolite extruded in and around the Jinan Basin at ca. 90 Ma and 89 Ma respectively. Basaltic trachyandesite and andesite extruded and intruded at ca. 84 Ma. The average depth of the magma chamber for andesite and basaltic trachyandesite is 9.6 km and 10.7 km, respectively. 3) The enrichement in incompatible trace element and Sr-Nd-Pb isotope geochemistry as well

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as the mafic nature (high Mg#, MgO and Cr) of the trachybasalt, basaltic trachyandesite and

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andesite in the Jinan Basin indicates that they originated from an enriched lithospheric mantle source and experienced crustal assimilation during ascent. The most likely contaminant is the

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Paleoproterozoic granitic gneiss in the Yeongnam Massif. These geochemical features are

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similar to those of the mid-Cretaceous mafic volcanic rocks in the Yeongdong and Gyeongsang Basins.

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4) During the mid-Cretaceous, the igneous rocks in the southern Korean Peninsula formed

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due to upwelling asthenospheric mantle related with back arc extension caused by the slab-

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rollback and steepening of the paleo-Pacific plate, which also occurred in the southeastern margin of the SCC between 140 and 85 Ma.

Acknowledgement We thank to Hyeon Ih Ryu at KBSI in Jeonju center for assistance with EPMA analysis and Youn-Joong Jeong in KBSI in Ochang for assistance with LA-MC-ICPMS analysis. We also thank Bo Young Lee and undergraduate students at Chonbuk national university for assistance with fieldwork. This study was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean Government (NRF-2017R1A2B2011224 and NRF2017K1A1A2013180)

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Declaration of Interest No conflict of interest exists.

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Korea. 49, 31-45. Reedman A. J. and S. H. Um, 1975. The Geology of Korea. Korea Institute of Energy and Resources, 139p. Sagong, H., Kwon, S.-T., Cheong, C.-S., Choi, S.H., 2001. Geochemical and isotopic studies of the Cretaceous igneous rocks in the Yeongdong Basin, Korea: Implication for the origin of magmatism in pull-apart basin. Geosciences Journal. 5, 191-201.

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Shin, I.-H., Park, C.-Y., Jeong, Y.-J., 2001. Petrochemistry and Sr·Nd isotopic composition

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of foliated granite in the Jeonju area, Korea. Journal of Korean Earth Science Society.

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Son, C.M., 1969. On the Cretaceous igneous activities in Korea. Journal of Geological Society of Korea. 5, 259-267. Sun, S.S., McDonough,W.F., 1989. Chemical and isotopic systematic of oceanic basalt: Implications formantle composition and processes. In: Saunders, A.D., Norry, M.J. (Eds.), Magmatism in the Ocean Basins. The Geological Society of London. 313–345. Sung, J.G., Kim, J.S., Lee, J.D., 1998. Petrochemical study on the Cretaceous volcanic rocks in Kyeongsang Basin, Korea: possibility of magma heterogeneity. Economic and Environmental Geology. 3, 249-264. Sung, J.G., Kim, J.S., 2012. Petrochemical characteristics and review on petrogenesis on Cretaceous to Tertiary volcanic rocks in the Kyongsang Basin. Journal of Petrological

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Society of Korea. 2, 217-233. Xu, J., Zhu, G., Tong, W.X., Gui, K.R., Liu, Q., 1987. Formation and evolution of the Tancheng-Lujiang wrench fault system: a major shear wywtem to the northwest of the Pacific Ocean. Tectonophysics. 134, 273-310. Xue, H., Tao, K., Shen, J., Sr and Nd isotopic characteristics and magma genesis of Mesozoic volcanic rocks along the coastal region of southeastern China. Acta Geological Sinica. 9,

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Figure caption

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Figure 1. (a) Simplified tectonic map of the East Asia (after Kim et al., 2016); (b) The

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distribution map of the Cretaceous basins and strike-slip fault system in the southern Korean peninsula. Abbreviations are as follow, KFS: Kongju-Eumsung fault system, GFS: Gwangju-

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Yeongdong fault system, 1: Tando basin, 2: Namyang basin, 3: Cheonsuman basin, 4:

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Pungam basin, 5: Eumsung basin, 6: Kongju basin, 7: Puyeo basin, 8. Kyokpo basin, 9.

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Gyehwa basin, 10. Yeongdong Basin, 11. Muju basin, 12. Jinan Basin, 13. Hampyeong basin, 14. Haenam basin, 15. Meungju basin, 16: Gyeongsang Basin (after Lee et al., 1999; Ryang, 2013).

Figure 2. (a) Tectonic map of South Korea (after, Lee e t al., 2016); (b) Geological map of the Jinan Basin (after, Lee, 1992; ; Kim et al., 1973; Kim et al., 1984) Abbreviations are as follow, GM: Gyeonggi Massif, OMB: Okcheon Metamorphic Belt, TB: Taebaeksan Basin, YM: Yeongnam Massif, GB: Gyeongsang Basin.

Figure 3. The outcrop photographs for the mid-Cretaceous volcanic rocks in the Jinan Basin.

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(a) Trachybasalt enclave (JA161015-1B) in the andesitic rock; (b) Basaltic trachyandesite (JA161015-1A); (c) Intrusive basaltic trachyandesite (JA151231-1A); (d) Andesite (JA151010-1B); (e) Rhyolite (JA151231-3F); (f) Tuff (JA151010-2L)

Figure 4. The photomicrographs showing the mineral assemblages and textures for the midCretaceous volcanic rocks in the Jinan Basin. (a) Trachybasalt enclave (JA161015-1B) in the

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andesitic rock; (b) Basaltic trachyandesite (JA161015-1A); (c) Intrusive basaltic trachyandesite (JA151231-1A); (d) Andesite (JA151010-1B); (e) Rhyolite (JA151231-3F); (f)

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Tuff (JA151010-2L). Pl = Plagioclase, Cpx = Clinopyroxene, Kfd = K-feldspar, Qtz = Quartz.

Figure 5. Chemical classification of the Cretaceous volcanic rocks in the Jinan Basin. (a)

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Total alkali vs. silica (TAS) classification diagram after Le Bas (1986); (b) K 2 O vs. SiO 2

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diagram, in which field boundaries are from Peccerillo and Tayloer (1976); (c) FeOtNa2O+K2O-MgO diagram after Irvine and Baragar (1971); (d) A/NK vs. A/CNK diagram

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after Maniar and Piccoli (1989).

Figure 6. Tectonic discrimination diagrams for the volca nic rocks in the Jinan, Yeongdong and Gyeongsang Basins. (a) Rb vs. Y+Nb diagram after Pearce et al. (1984); (b) FeOt/MgO vs. Zr+Nb+Ce+Y diagram after Whalen et al. (1987); (c) Nb-Y-Ce diagram after Eby (1992); (d) V vs. Ti/1000 diagram after Shervais (1982); (e) Zr/Y vs. Zr diagram after Pearce and Norry (1979); (f) Cr vs. Y diagram after Pearce (1982). MORB: mid-ocean ridge basalt, BABB: back-arc basin basalt, FG: fractionated felsic granite, OGT: unfractionated M-, I-, Stype granites. GB: Gyeongsang Basin, YB: Yeongdong Basin. Data sources: Yeongdong Basin (Sagong et al., 2001); mid-Cretaceous Gyeongsang Basin (Kwon et al., 2013); mid to late-Cretaceous Gyeongsang Basin (Hwnag and Kim, 1994a,b; Kim and Yun, 1993; Kim et

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al., 1998; Yun et al., 1997; Sung et al., 1998; Sung and Kim, 2012); Jinan Basin (Kim, 2015, this study).

Figure 7. Chondrite- normalized REE and primitive mantle-normalized multi-element patterns for trachybasalt (a), (b), basaltic trachyandesite and andesite (c), (d) and rhyolite (e),(f) in the Jinan, Yeongdong and Gyeongsang Basins. These diagrams are normalized to the chondrite

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and primitive mantle compositions suggested by Sun and McDonough (1989). The

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Nd/144Nd vs.

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Sr/88 Sr isotopic ratios for the Cretaceous volcanic rocks in the

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Figure 8.

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abbreviations in the Figure 6 are used.

Jinan, Yeongdong and Gyeongsang Basin and basement rocks around the Jinan Basin. Data

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for Jurassic granite and Paleoproterozoic granitic gneiss are from Shin et al. (2001) and Lee

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et al. (2005), respectively. The field areas for the MORB and mantle array are from Zinder and Hart (1986). The isotope rations for Triassic basement rocks of the Gyeongsang Basin

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(Cheong et al., 2002) are calculated back to 100Ma by Kwon et al. (2013). The isotope data

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for mid-Cretaceous basalt and rhyolite in the Yeongdong Basin and basalt in the Gyeongsang Basin are from Sagong et al., (2001) and Kwon et al., (2013) respectively. Data sources of Cretaceous basalt and rhyolite from the southeastern SCC are from He and Xu, (2012) a nd references therein. Mixing lines are shown between the Cretaceous basalt in the southeastern SCC (Liao et al., 1999) and the Paleoproterozoic granitic gneiss.

Figure 9. (a)

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Pb/204 Pb vs.

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Pb/204 Pb and (b)

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Pb/204Pb vs.

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Pb/204Pb for the Cretaceous

volcanic rocks in the Jinan Basin. Gray field area represents isotope ratios of MORB (Zindler and Hart, 1986), NHRL (Northern Hemisphere reference line) are from Hart (1984) and the field of the Yeongnam Massif and Gyeongsang Basin are from Kwon et al. (2013). DMM:

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depleted mantel, EM1: enriched mantle type 1, EM2: enriched mantle type 2, HIMU: mantle with high U/Pb ratio.

Figure 10. The scanning electron microscopy (SEM) cathodoluminescence (CL) images of the zircons from the lowest sandstone in the Sansudong formation (a), the uppermost sandstone in the Dalgil formation (b), basaltic trachyandesite (c, d), andesite (e), rhyolite (f, g) 206

Pb/238 Pb ages.

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in the Jinan Basin. Circles and numbers represent analyzed spots and

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Figure 11. Concordia diagrams showing the results of U-Pb isotopic analyses on the zircons

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using LA-MC-ICP-MS from the lowest sandstone in the Sansudong formation (a), the

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rhyolite (f, g) in the Jinan Basin.

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uppermost sandstone in the Dalgil formation (b), basaltic trachyandesite (c, d), andesite (e),

Figure 12. Binary plots of Mg# vs. SiO 2 (a), MgO vs. SiO 2 (b), Cr vs. SiO 2 (c), CaO/Al2 O3 vs.

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K2O/MgO(d) for the mafic to intermediate volcanic rocks in the Jinan Basin. Fields for the

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dehydration and H2 O-added melting of lower crust experiments and experimental melt are from Lu et al. (2015) and Couzinié et al. (2016) and references therein.

Figure 13. Binary plots of (a) Rb/Nb vs. Th/Y and (b) Ce/Pb vs. Nb/U for the mafic to intermediate volcanic rocks in the Jinan Basin. Solid lines represent trace element mixing models between the Cretaceous basalt in the Southeastern SCC (Lapierre et al., 1997) and Paleoproterozoic granitic gneiss in the Yeongnam Massif (Lee et al., 2019).

Figure 14. Tectonic discrimination diagrams for the Cretaceous basalt and rhyolite in the southeastern margin of SCC. Data are from Chen and Zhou, (1995), Lapierre et al., (1997),

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Liao et al., (1999), Chen et al., (2000) and Zhou et al., (2015)

Figure 15. (a) The simple tectonic map of the Northeast Asia during Cretaceous showing propagation of the igneous activity related to back arc extension from the southern part of the eastern margin of the SCC at 140 Ma to the Korean Peninsula at 110-80 Ma (modified after Zhou and Li, 2000). (b) The flat subduction without volcanism before 110 Ma and (c) the

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igneous activity related to back arc extension caused by the upwelling asthenospheric mantle

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due to slab roll-back during 110-80 Ma in the Korean Peninsula.

Supplementary table

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Supplementary table 1. The representative composition of plagioclase in the Jinan Basin

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Supplementary table 2. The representative composition of clinopyroxene in the Jinan Basin

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Supplementary table 3. Whole rock composition of the trachybasalt, basaltic trachyandesite,

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andesite and rhyolite in the Jinan Basin

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Supplementary table 4. Whole rock isotope composition of the basaltic trachyandesite and andesite in the Jinan Basin

Supplementary table 5. LA-MC-ICPMS zircon ages of the sandstones and volcanic rocks in the Jinan Basin Supplementary table 6. Pre-eruptive temperature and pressure of basaltic trachyandesite and andesite in the Jinan Basin

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Highlights:

 The Jinan Basin locates in the southern Korean Peninsula (SKP).  Volcanic activity in the Jinan Basin occurred during mid-Cretaceous (midC).

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 Mid-C igneous activities in SKP formed due to back-arc extension.

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 The extension caused asthenospheric mantle upwelling supplying heat.

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 Mid-C volcanic rocks in SKP formed by partial melting of enriched SCLM.

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