2019 | 72 / Special Issue | 19–32 | 15 Figs. | 3 Tabs. | www.geologia-croatica.hr Journal of the Croatian Geological Survey and the Croatian Geological Society Article history: Manuscript received May 13, 2019 Revised manuscript accepted August 28, 2019 Available online December 20, 2019 Keywords: Diabase, Ganzi-Litang Ocean, Luoji, Early Permian, Western Yunnan 1. INTRODUCTION Continental rift basalt (CRB) is one of the key geological records for understanding the mechanism and process of continental rift- ing (e.g., FREY et al., 1978; SUN & MCDONOUGH, 1989; MAC- DONALD et al., 2001; CUI et al., 2015; MA et al., 2017). In rift environments, due to the interaction between tectonics and mag- matism, the continental lithosphere usually undergoes substantial thinning, which inevitably leads to the upwelling of mantle magma (HAWKESWORTH et al., 1993; WANG et al., 2002; CUI Geochemistry and U-Pb ages of the diabases from the Luoji area, western Yunnan, China: implications for the timing of the initial rifting of the Ganzi-Litang Ocean Chan Wang1,2, Hao Liu2,3,*, Hanbin Feng4, Jianghong Deng1, Xianfan Liu1 and Fufeng Zhao1 1 Chengdu University of Technology, College of Earth Sciences, Chengdu, Sichuan, 610059, China 2 School of Environment and Earth Science, University of Queensland, Brisbane, QLD, 4072, Australia 3 Chengdu University of Technology, College of Energy, State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu Sichuan, 610059, China 4 North China Petroleum Administration Co., Ltd. Sulige Exploration and Development Branch, Shijiazhuang, Hebei, 017300, China (*correspondence author: Hao Liu, 277515537@qq.com) doi: 10.4154/gc.2019.25 Abstract Detailed geochemical and U-Pb studies of two diabases (Luoji and Cuiyi) from the Luoji area have been undertaken. The diabases are high-K calc-alkaline and belonging to the tholeiitic se- ries, enriched in large ion lithophile elements, Ti, Zr and light rare earth elements, and depleted in high field strength elements. These characteristics are different from the oceanic island basalt but highly consistent with the continental rift basalt, indicating the Luoji and Cuiyi diabases are the products of the intracontinental rift related to the initial opening of the Ganzi-Litang Ocean. The Luoji and Cuiyi diabases originated from an enriched mantle source with a small degree of crustal contamination during their emplacement. Zircon U-Pb ages show that the Luoji and Cuiyi diabases were emplaced at 293.4±5.4Ma. Therefore, we propose that the time of initial rifting of the Ganzi-Litang Ocean occurred during the very Early Permian. et al., 2015; KHOSHNOODI et al., 2017; ZHOU et al., 2018). Con- sequently, at the beginning stage of rifting, mantle-derived mag- mas with signatures of CRB will emplace along the tensioned weak zone of a continental rift belt (MACDONALD et al., 2001; CUI et al., 2015; SZABÓ et al., 2016). A CRB is crucial to the study of the continental within-plate tectonic movement and the initial evolution of the ocean basin as it has a fundamental role in constraining the initiation time of the ocean opening (LASSITER & DEPAOLO, 1997; JUNG & MASBERG, 1998; CUI et al., 2015). Figure 1. Location of the western Yunnan in China (a), geotectonic location map (b) and regional geological map of the Luoji diabase in western Yunnan province (c) (after HOU et al., 2001). G eo lo gi a C ro at ic a Geologia Croatica 72 / Special Issue20 The initial opening time of the Ganzi-Litang Ocean has al- ways been controversial. In order to reveal the process of tectonic evolution with the Ganzi-Litang Ocean, mafic diabases developed in the Ganzi-Litang suture zone of Tibet have been extensively studied by earlier researchers (e.g., HOU et al., 2001; YBGMR- RGST, 2003; LIANG et al., 2018). However, most of the reported diabases are oceanic island basalts (OIB) or mid-ocean ridge ba- salts (MORB) (HOU et al., 2001). Hence, the time of initial rift- ing of the Ganzi-Litang Ocean is still not well constrained. In this research, we conducted whole-rock geochemistry and zircon U-Pb dating analysis of two diabases (Luoji and Cuiyi) from the Luoji area, the southern segment of the Ganzi-Litang suture zone. The diabases in this study show typical CRB char- acteristics, hence can be used to constrain the initial evolution of the Ganzi-Litang Ocean. 2. GEOLOGICAL BACKGROUND AND LITHOFACIES CHARACTERISTICS The Luoji and Cuiyi diabases are located in the Luoji Country in the northwestern part of Shangri-La County, Yunnan Province, SW China. They are a part of the Ganzi-Litang Ocean basin and located at the junction of the western margin of the Yangtze block, the Zhongzan-Zhongdian continental block and the Yidun island arc belt (Fig. 1a). Magmatic activities in this area mainly occurred dur- ing Permian to Triassic time (HOU et al., 2001; YBGMRRGST, 2003; LIANG et al., 2018). Most of the ultrabasic-basic rocks have been deformed and transformed due to multi-stage tectonic move- ments. During the continental collision stage, most of the ultrabasic- basic rocks occur in the form of a mélange (YBGMRRGST, 2003). The ultrabasic rocks in the study area occur in the Lower Permian Mushengtu Formation. Most of the basic rocks are dia- Figure 2. (a) Outcrop of diabase with a diabasic texture; (b) Photomicrograph of diabase (cross-polarized light). Ilm-ilmenite; Pl-plagioclase; Di-diopside; Aug-au- gite; Q-quartz. Figure 3. Zircon U-Pb concordia diagrams for the Luoji diabase. G eologia C roatica Wang et al.: Geochemistry and U-Pb ages of the diabases from the Luoji area, western Yunnan, China: implications for the timing of the initial rifting ... 21 base dykes cropping out in the Lapoli, Sijiagou and Zhangjia ar- eas. The dykes are distributed in NW-SE direction with a width ranging from tens of centimetres to a few metres and they extend for hundreds of metres. The diabases intruded into the Mush- engtu Formation and the Lower Permian Luoji Formation (YB- GMRRGST, 2003). The Luoji and Cuiyi diabases are dark gray-grayish green (Fig. 2a), fine- to medium-grained (0.2~2mm.) with a diabasic texture. The rocks are composed of plagioclase (75~79%), augite (5~8%), diopside (2~5%) and quartz (1%) with accessory mine- rals of apatite, magnetite and ilmenite (Fig. 2b). 3. SAMPLES AND METHODS Rock samples for this study were collected from outcrops. Only the fresh to least-altered samples were selected on the basis of hand lens observations for further analysis by thin- section. Major and trace elements were analyzed at the Metallurgical Geological Rocks and Ore testing Center, Sichuan, China. For major element analyses, powder samples (grain size less than 200 mesh) were fluxed with Li2B4O7 (1:8) to make homogeneous glass discs at 1,250°C using a V8C automatic fusion machine. The bulk rock major elements were analyzed on fused glass discs with a Zetium (PANalytical) sequential X-ray fluorescence spectrome- ter. The loss on ignition (LOI) of sample powders was determined at a temperature of 1,000°C. The measurements were monitored using the standards GSR-3 (XIE et al., 1989) and BHVO-2 (RAC- ZEK et al., 2010), and the accuracy of the analytical results was controlled by measuring the standard reference material BHVO- 2. The analytical errors for the major element analyses were bet- ter than 2%. Trace element analyses were conducted using inductively coupled plasma mass spectrometry (ICP-MS) (Thermal X Series II). During the digestion procedure, 25 mg of sample powder was precisely weighed and transferred into screw-top Teflon beakers. Then 1.5 ml HF and 1.5 ml HNO3 were added in turn and heated in the closed Teflon beakers at 100°C for 96 hours, subsequently opened and heated at 120°C to evaporate the dissolution. Then, 0.5 ml HF and 2.5 ml 1:1 HNO3 were added, and the beakers were transferred to a digestion bomb. The bombs were heated in an oven at 170°C for 72 hours, after which the beakers were opened and heated at 120°C to evaporate the dissolutions. The residues were added to 1:1 HNO3, sealed and heated at 120°C for 30 min. Figure 4. (a) SiO2 (wt.%) vs. Na2O+K2O (wt.%) diagram (after PEARCE, 1979); (b) SiO2 (wt.%) vs. K2O (wt.%) diagram (after MANIAR & PICCOLI, 1989); (c) Whole-rock alkali (Na2O+K2O) vs. TFeO vs. MgO (AFM) plot for Luoji and Cuiyi samples (after IRVINE & BARAGER, 1971). G eo lo gi a C ro at ic a Geologia Croatica 72 / Special Issue22 The solutions were then diluted 1,000 times using 2% distilled super-pure HNO3 and analyzed using ICP-MS with In, Rh and Re as inner standards. The standard solutions (American Lab Tech Company) were diluted to 1 μg l-1, 10 μg l-1, 50 μg l-1, and 100 μg l-1 to produce the calibration curve, with linear regression coefficients all above 0.9999. The BHVO-2 and GSR-3 were run as external standards to evaluate the accuracy. Analytical preci- sion for most elements is better than 3%. In order to constrain their emplacement time, zircon grains were handpicked from the diabases under a binocular microscope after conventional heavy liquid and magnetic separation, and then mounted in an epoxy resin. The sample mount was polished to expose the centres of the zircons. All of the grains were photo- graphed under transmitted and reflected light and then examined using cathodoluminescence (CL) imaging to observe the internal structures of the zircons. U-Pb analyses of the zircons were car- ried out using an Agilent Technologies 7700x quadrupole ICP- MS equipped with a 193 nm ArF excimer laser at the Institute of Geophysical Geochemistry, Chinese Academy of Geological Sci- ences, Beijing. The laser beam was focused on the sample with a fluency of 5.0 J/cm2 and a spot of 40 μm diameter at a repetition rate of 5 Hz for 50 s. Helium was used as a carrier gas to trans- port the ablated aerosol to the mass-spectrometer. Zircon Nancy 91500 (WEDENBECK et al., 1995) was used as an external cali- bration standard to correct for instrumental mass bias and ele- mental fractionation. The standard GJ-1 (JACKSON et al., 2004) was simultaneously analyzed which yielded ages consistent with the recommended values within analytical error. The Pb content of the zircon was externally calibrated against NIST SRM 610 with Si as an internal standard, whereas other trace elements were measured with Zr as an internal standard (HU et al., 2011). Raw- data reduction was performed off-line using the ICPMSDataCal (LIU et al., 2010). Concordia diagrams and weighted average cal- culations were made using Isoplot 3.0 (LUDWIG, 2012). Uncer- tainties in individual analyses are reported at the 1σ level, and the average ages for pooled U-Pb analyses are quoted at 95% confi- dence. 4. RESULTS 4.1. Zircon U-Pb dating Cathodoluminescence (CL) images of representative zircon grains are shown in Figure 3. and LA-ICP-MS zircon U-Pb data are presented in Table 1. Twenty-one zircon grains from the dia- bases are mostly euhedral to subhedral, with lengths of 40-100μm and length-to-width ratios of 1:1 to 3:1, and have weak oscillatory zones indicating a magmatic origin. Zircon grains from the Luoji diabase have U contents of 215–1502 ppm (average 647.29), Th contents of 45–5824 ppm (average 1421.67) and Th/U ratios from 0.11 to 5.80 (Table 3), indicating they are of magmatic origin. The single spot ages vary from 1907±17Ma to 291±4Ma with three distinctive peaks of 1245Ma~1028Ma, 901Ma~757Ma and 303Ma~291Ma. For the youngest set of data, the spots yield a 206Pb/238U weighted mean age of 293.4±5.4Ma (Fig. 3b, MSWD=1.7, N=5), which indicate that the diabase was emplaced during the Early Permian. 4.2. Major elements As shown in Table 2, the LOI values of the Luoji and Cuiyi dia- bases are between 0.41% and 4.87%. The Luoji diabase may be slightly affected by alteration, so the normalized calculation is carried out after deducting the LOI (IRVINE & BARAGAR, 1971). Luoji diabase samples contain 48.86~50.89 wt.% SiO2 (ave rage 49.68 wt.%), 1.28~2.09 wt.% K2O (average 1.57 wt.%), 2.55~3.36 wt.% Na2O (average 2.90 wt.%), 0.16~0.43 wt.% P2O5 (average 0.33 wt.%), 3.68~4.01 wt.% TiO2 (average 3.82 wt.%), 9.42~13.06 wt.% FeO (average 10.51 wt.%) and 4.17~6.12 wt.% MgO (average 5.11 wt.%) with high Na2O/K2O (1.88 on average) and FeO/MgO (2.10 on average) ratios, and low Mg# values of 0.40~0.52 (0.46 on average). Samples of Cuiyi diabase contain relatively lower SiO2 (46.95~48.63 wt.%, 47.96 wt.% on average), K2O of 0.68~1.42 wt.% (average 1.14 wt.%), TiO2 of 2.56~2.79 wt.% (average 2.64 wt.%), FeO of 7.81~9.27 wt.% (average 8.46 wt.%), and higher Na2O of 2.97~3.60 wt.% (average 3.23 wt.%), P2O5 of 0.13~0.56 wt.% (average 0.34 wt.%), and slightly high Na2O/K2O (3.03 on average) and FeO/MgO (1.48 on average) ra- tios, MgO of 5.16~6.34 wt.% (average 5.78 wt.%) and Mg# values of 0.50~0.57 (average 0.55) than Luoji diabase samples. In the SiO2 vs. K2O+Na2O diagram (Fig. 4a), all samples plot in the basalt field and are distributed along the line of the alka- line-subalkaline series. They are calc-alkaline to high-K calc- alkaline in composition (Fig. 4b) and belong to tholeiite types (Fig. 4c). The high P2O5 contents of the Luoji and Cuiyi diabases indicate their magma may originate from an enriched mantle (CAMPBELL & GRIFFITHS, 1993). 4.3. Rare earth elements Rare earth element compositions of the samples in the study are shown in Table 3. Total REE values of the diabase samples range from 163.65 ppm to 301.07 ppm (average 219.90 ppm) with LREE/ HREE ratios of 6.69~9.52. These samples are enriched in LREE and show high LREE/HREE fractionation ((La/Yb) N=11.05~12.29, Fig. 5a). The normalized REE distribution pat- terns are similar to the CRB (CUI et al., 2015). The samples show negligible negative to slight positive Eu anomalies (δEu=0.93~1.06), suggesting the existence of pyroxene fractional crystallization (SUN & MCDONOUGH, 1989; WEAVER, 1991; MIDDLEMOST, 1994). 4.4. Trace elements The trace element results are shown in Table 4. On the primitive mantle-normalized diagram (Fig. 5b), the samples show enrich- ment in the large ion lithophile elements (LILEs) of Rb, Ba and K and high field strength elements (HFSEs) of Th, Ta and Ti, and depletion in Sr, U, Nb, P and Zr. The reason for a slight depletion in K in all samples relative to Ba and Rb is that Ba and Rb are prone to enter into K-bearing phases during the processes of mag- matic fractional crystallization and differentiation (FREY et al., 1978; SZABÓ et al., 2016). The ratios of Zr/Nb (5.76~6.23), La/ Nb (0.95~1.00), Th/Nb (0.11~0.13) and K/Nb (255.51~334.98) in the Luoji and Cuiyi diabases are similar to the CRB but are dis- tinct from the OIB (Fig. 5b). 5. DISCUSSION 5.1. Special tectonic setting of the Continental Rift Basalt (CRB) Based on tectonic settings, basalts can be divided into three types: Continental Flood Basalts (CFB), Mid-Ocean Ridge Ba- salts (MORB), and Ocean Island Basalt (OIB) (GREENOUGH & MCDIVITT, 2018). Different tectonic settings will produce dia- bases with distinct characteristics. Hence, we can identify and discuss the environment in which diabase occurred with these unique characteristics. Major element contents of all samples are G eologia C roatica Wang et al.: Geochemistry and U-Pb ages of the diabases from the Luoji area, western Yunnan, China: implications for the timing of the initial rifting ... 23 Ta bl e 1. L A -IC P- M S U -P b is ot op ic d at a fo r g ab br o in th e Lu oj i o ph io lit e su ite . Sp ot Pb U Th (p pm ) Th /U 20 7 Pb /20 6 Pb 20 7 Pb /23 5 U 20 6 Pb /23 8 U 20 7 Pb /20 6 Pb 20 7 Pb /23 5 U 20 6 Pb /23 8 U (p pm ) (p pm ) Ra tio 1σ Ra tio 1σ Ra tio 1σ Ag e( M a) 1σ Ag e( M a) 1σ Ag e( M a) 1σ 1 34 32 2 31 4. 36 9 0. 97 6 0. 05 83 0. 00 06 0. 70 96 0. 01 0. 08 82 0. 00 09 54 2 22 54 4 8 54 5 5 2 96 10 30 37 54 .4 53 3. 64 5 0. 06 16 0. 00 31 0. 39 22 0. 01 99 0. 04 62 0. 00 04 66 1 10 8 33 6 17 29 1 2 3 88 73 2 42 43 .8 43 5. 79 8 0. 05 35 0. 00 09 0. 35 54 0. 00 83 0. 04 82 0. 00 08 35 1 37 30 9 7 30 3 5 4 85 11 36 41 14 .9 33 3. 62 2 0. 05 87 0. 00 24 0. 38 17 0. 01 8 0. 04 71 0. 00 09 55 7 88 32 8 15 29 7 5 5 11 9 12 54 58 23 .8 27 4. 64 4 0. 05 45 0. 00 1 0. 34 7 0. 00 7 0. 04 62 0. 00 06 39 1 42 30 2 6 29 1 4 6 11 8 13 92 39 37 .4 11 2. 82 9 0. 05 43 0. 00 02 0. 35 31 0. 00 47 0. 04 72 0. 00 06 38 2 10 30 7 4 29 7 4 7 15 6 65 2 66 9. 34 3 1. 02 7 0. 08 61 0. 00 05 2. 52 89 0. 01 83 0. 21 31 0. 00 17 13 40 11 12 80 9 12 45 10 8 56 43 4 45 .3 53 0. 10 5 0. 06 58 0. 00 07 1. 13 16 0. 01 19 0. 12 47 0. 00 11 80 1 22 76 9 8 75 7 7 9 20 23 5 17 2. 67 8 0. 73 5 0. 05 67 0. 00 1 0. 60 29 0. 01 11 0. 07 71 0. 00 06 48 0 39 47 9 9 47 9 4 10 15 1 59 5 79 .4 33 0. 13 4 0. 09 54 0. 00 13 3. 41 23 0. 05 12 0. 25 94 0. 00 26 15 36 25 15 07 23 14 87 15 11 55 28 2 11 9. 34 2 0. 42 3 0. 07 71 0. 00 07 2. 03 26 0. 02 65 0. 19 13 0. 00 17 11 23 18 11 26 15 11 28 10 12 37 21 5 20 1. 75 6 0. 93 8 0. 07 15 0. 00 17 1. 46 09 0. 03 83 0. 14 81 0. 00 14 97 3 49 91 4 24 89 0 8 13 98 48 4 92 8. 21 5 1. 91 8 0. 07 06 0. 00 27 1. 44 11 0. 05 69 0. 14 81 0. 00 15 94 5 77 90 6 36 89 0 9 14 59 42 1 13 33 .9 81 3. 16 9 0. 05 69 0. 00 06 0. 65 61 0. 00 72 0. 08 37 0. 00 03 48 6 23 51 2 6 51 8 2 15 24 33 5 14 3. 98 3 0. 43 0 0. 06 36 0. 00 4 0. 50 42 0. 03 28 0. 05 75 0. 00 06 72 7 13 4 41 5 27 36 1 4 16 25 8 15 02 20 57 .7 40 1. 37 0 0. 07 32 0. 00 05 1. 51 38 0. 01 4 0. 14 99 0. 00 12 10 20 15 93 6 9 90 1 7 17 58 35 7 52 6. 75 4 1. 47 6 0. 06 38 0. 00 07 1. 09 71 0. 01 36 0. 12 48 0. 00 15 73 4 23 75 2 9 75 8 9 18 13 7 70 8 57 7. 79 9 0. 81 6 0. 08 14 0. 00 05 1. 94 19 0. 01 25 0. 17 29 0. 00 13 12 32 11 10 96 7 10 28 8 19 10 5 47 6 25 1. 13 8 0. 52 8 0. 09 01 0. 00 15 2. 51 59 0. 05 14 0. 20 25 0. 00 23 14 28 32 12 77 26 11 89 13 3 20 23 9 75 0 29 2. 20 0 0. 39 0 0. 11 64 0. 00 1 5. 16 33 0. 05 14 0. 32 18 0. 00 32 19 01 15 18 47 18 17 99 18 21 12 0 28 1 26 8. 46 7 0. 95 5 0. 15 2 0. 00 07 7. 21 57 0. 07 15 0. 34 43 0. 00 3 23 69 8 21 38 21 19 07 17 Ta bl e 2. M aj or e le m en ts d at a (w t.% ) f or d ia ba se d yk es . LJ -1 LJ -2 LJ -3 LJ -4 LJ -5 LJ -6 LJ -7 LJ -8 LJ -9 CY -1 CY -2 CY -3 CY -4 CY -5 CY -6 CY -7 Si O 2 47 .3 5 46 .8 5 46 .3 2 46 .5 7 47 .6 2 48 .6 2 47 .5 9 46 .9 2 48 .2 1 47 .2 1 46 .4 1 48 .4 3 48 .1 7 46 .9 7 47 .8 9 48 .3 4 Ti O 2 3. 51 3. 68 3. 72 3. 64 3. 54 3. 71 3. 51 3. 65 3. 84 2. 56 2. 76 2. 57 2. 61 2. 59 2. 68 2. 58 A l 2O 3 13 .2 1 14 .2 3 13 .6 7 12 .7 3 13 .2 2 12 .5 7 13 .1 8 13 .9 9 12 .4 9 15 .4 6 15 .1 4 14 .2 1 15 .0 9 14 .9 8 14 .9 7 13 .9 9 Fe 2O 3 2. 43 2. 54 1. 97 1. 84 4. 42 2. 43 4. 37 2. 67 2. 59 6. 47 7. 01 6. 79 4. 78 6. 52 4. 86 6. 82 Fe O 9. 52 9. 83 11 .6 2 12 .4 4 8. 96 9. 61 8. 99 9. 84 9. 78 7. 82 7. 65 9. 13 8. 24 7. 86 8. 29 9. 15 M nO 0. 21 0. 22 0. 24 0. 23 0. 23 0. 23 0. 22 0. 19 0. 23 0. 13 0. 12 0. 09 0. 07 0. 15 0. 12 0. 09 M gO 5. 41 4. 76 4. 74 4. 62 3. 97 5. 79 4. 05 4. 59 5. 86 5. 88 5. 41 5. 12 5. 99 5. 91 6. 22 5. 09 Ca O 9. 34 9. 38 8. 58 8. 81 7. 94 7. 64 7. 99 8. 97 7. 94 9. 02 9. 68 8. 71 9. 74 9. 11 9. 75 9. 04 N a 2 O 2. 71 2. 61 2. 41 2. 43 2. 85 3. 21 2. 81 2. 63 3. 17 3. 24 2. 94 3. 09 3. 57 3. 17 3. 41 3. 03 K 2 O 1. 29 1. 23 1. 21 1. 61 1. 99 1. 57 1. 87 1. 27 1. 46 1. 15 1. 19 1. 42 0. 67 1. 16 1. 05 1. 26 P 2 O 5 0. 37 0. 34 0. 31 0. 36 0. 41 0. 15 0. 36 0. 32 0. 19 0. 35 0. 55 0. 53 0. 13 0. 32 0. 21 0. 27 LO I 4. 23 4. 07 4. 87 4. 62 4. 52 3. 94 4. 48 3. 98 3. 87 1. 57 1. 68 0. 41 1. 48 1. 62 1. 36 1. 18 To ta l 99 .5 8 99 .7 4 99 .6 6 99 .9 99 .6 7 99 .4 7 99 .4 2 99 .0 2 99 .6 3 10 0. 86 10 0. 54 10 0. 5 10 0. 54 10 0. 36 10 0. 81 10 0. 84 M g# 0. 50 33 0. 46 33 0. 42 1 0. 39 84 0. 44 13 0. 51 79 0. 44 54 0. 45 4 0. 51 65 0. 57 27 0. 55 77 0. 49 99 0. 56 45 0. 57 27 0. 57 22 0. 49 79 N ot ic e: M g# =M g2+ /( M g2+ + Fe 2+ ) G eo lo gi a C ro at ic a Geologia Croatica 72 / Special Issue24 Ta bl e 3. Ta ce e le m en ts (p pm ) a nd R ar e ea rt h el em en ts (p pm ) c om po si tio ns o f L uo ji an d Cu iy i d ia ba se . LJ -1 LJ -2 LJ -3 LJ -4 LJ -5 LJ -6 LJ -7 LJ -8 LJ -9 CY -1 CY -2 CY -3 CY -4 CY -5 CY -6 CY -7 Rb 32 .9 8 34 .5 2 43 .4 8 41 .9 8 58 .6 9 39 .1 8 33 .9 7 36 .9 7 38 .6 7 30 .6 5 22 .6 1 24 .9 9 36 .9 2 29 .6 1 31 .7 6 26 .7 6 Cr 12 9. 46 97 .9 6 14 6. 92 86 .9 7 49 .2 3 40 .6 8 12 6. 46 96 .5 1 41 .5 2 75 .8 6 11 3. 58 46 .3 7 15 9. 34 16 0. 32 11 6. 79 55 .6 2 N i 80 .4 9 75 .8 2 90 .5 3 77 .6 8 48 .5 7 50 .1 7 81 .7 5 73 .2 9 46 .8 3 64 .9 1 87 .2 3 63 .5 9 62 .1 9 57 .8 6 75 .6 8 52 .6 4 V 44 8. 87 57 0. 94 54 7. 61 50 3. 46 42 6. 94 58 1. 59 43 2. 89 56 8. 14 48 3. 95 38 2. 68 31 5. 69 28 7. 46 32 2. 58 16 9. 52 36 9. 32 27 9. 51 Ba 34 8. 51 42 9. 03 43 6. 52 49 0. 16 59 7. 01 54 8. 26 34 9. 51 43 5. 68 51 8. 46 61 3. 92 65 7. 41 29 0. 73 83 5. 67 60 1. 39 34 6. 12 28 5. 67 Th 4. 31 4. 91 4. 69 5. 08 5. 63 5. 34 4. 37 4. 89 5. 39 2. 91 3. 71 3. 16 6. 09 7. 36 3. 51 3. 12 U 0. 9 1. 01 0. 98 1. 05 1. 12 1. 05 0. 92 1. 15 1. 08 1. 25 0. 73 0. 59 1. 58 1. 58 1. 29 0. 62 Ta 2. 32 2. 55 2. 54 2. 71 3. 04 3. 19 2. 36 2. 63 3. 24 3. 02 1. 75 1. 96 2. 34 3. 74 3. 04 1. 98 N b 34 .9 7 39 .1 7 39 .4 6 42 .7 9 49 .0 8 43 .5 8 33 .5 9 36 .9 4 44 .6 4 47 .5 9 28 .9 9 33 .8 6 50 .7 4 63 .8 4 45 .8 7 47 .3 6 Sr 51 1. 93 63 2. 13 57 4. 91 67 7. 62 87 0. 15 81 6. 74 50 9. 67 63 4. 15 84 6. 75 45 8. 76 18 3. 74 20 5. 31 63 9. 42 36 6. 21 46 1. 85 20 3. 46 Zr 21 2. 67 23 1. 59 22 6. 82 26 5. 99 29 1. 64 29 6. 81 21 6. 37 22 5. 98 29 7. 51 33 6. 54 22 3. 67 23 6. 53 36 1. 52 42 9. 76 33 7. 13 21 5. 28 H f 6. 31 6. 87 6. 81 7. 48 8. 17 8. 01 6. 28 6. 83 7. 68 8. 23 7. 39 7. 11 8. 97 9. 48 8. 94 8. 07 Y 25 .4 9 27 .5 9 28 .7 3 30 .5 8 33 .2 8 31 .9 7 24 .8 3 25 .4 9 30 .5 1 28 .1 7 29 .4 6 24 .1 6 36 .5 4 39 .6 1 29 .4 6 25 .4 9 Pb 4. 24 5. 62 4. 51 5. 4 5. 44 7. 07 5. 13 4. 96 6. 31 5. 23 4. 65 5. 32 4. 98 6. 17 5. 49 4. 82 Cs 0. 39 0. 64 0. 63 0. 73 0. 63 0. 97 0. 86 0. 71 0. 69 0. 57 0. 82 0. 74 0. 64 0. 58 0. 61 0. 73 La 36 .1 3 39 .2 7 39 .3 7 42 .6 9 47 .5 3 47 .0 8 37 .0 1 38 .9 9 46 .9 6 44 .3 4 30 .9 7 32 .8 6 49 .5 4 60 .9 5 50 .5 4 35 .7 9 Ce 76 .8 9 84 .7 5 86 .2 1 90 .4 8 10 0. 12 10 0. 18 76 .8 2 83 .9 6 99 .8 6 10 5. 42 62 .9 9 63 .7 3 11 2. 91 14 3. 67 11 7. 56 65 .8 1 Pr 9. 69 10 .6 7 10 .6 8 11 .6 5 12 .8 6 12 .8 7 9. 72 10 .5 3 12 .7 9 12 .4 6 7. 93 8. 07 12 .6 1 15 .0 7 13 .0 7 7. 96 N d 39 .1 2 42 .1 6 42 .7 5 47 .3 1 51 .6 9 51 .5 8 39 .0 5 42 .0 4 50 .8 3 34 .2 5 32 .0 6 29 .7 33 .3 6 38 .2 4 33 .9 7 28 .7 9 Sm 7. 42 8. 07 8. 23 8. 72 9. 35 9. 48 7. 35 8. 09 9. 53 9. 77 6. 39 7. 13 9. 96 11 .2 7 10 .6 4 6. 25 Eu 2. 27 2. 56 2. 47 2. 58 2. 86 2. 96 2. 34 2. 63 2. 88 2. 76 2. 04 1. 99 2. 91 3. 24 3. 16 2. 03 G d 6. 67 7. 24 7. 62 7. 79 8. 42 8. 72 6. 59 7. 16 8. 84 8. 49 6. 06 6. 18 8. 4 9. 52 8. 42 5. 86 Tb 1. 04 1. 12 1. 12 1. 16 1. 28 1. 27 1. 07 1. 15 1. 21 1. 34 1. 01 1. 12 1. 32 1. 47 1. 37 1. 21 D y 5. 87 6. 09 6. 47 6. 72 7. 18 7. 23 5. 82 6. 07 7. 22 7. 28 6. 08 6. 63 7. 3 8. 09 7. 42 5. 78 H o 1. 02 1. 12 1. 11 1. 15 1. 24 1. 22 1. 06 1. 09 1. 19 1. 3 1. 14 0. 91 1. 31 1. 43 1. 34 0. 92 Er 2. 59 2. 94 3. 08 3. 18 3. 43 3. 49 2. 61 2. 97 3. 48 3. 45 3. 17 2. 62 3. 4 3. 85 3. 48 2. 58 Tm 0. 36 0. 38 0. 42 0. 43 0. 49 0. 46 0. 32 0. 41 0. 45 0. 5 0. 48 0. 41 0. 48 0. 56 0. 49 0. 4 Yb 2. 34 2. 53 2. 55 2. 79 2. 95 2. 85 2. 29 2. 58 2. 82 2. 95 2. 88 2. 54 2. 83 3. 22 2. 54 2. 57 Lu 0. 35 0. 37 0. 43 0. 41 0. 46 0. 49 0. 34 0. 36 0. 48 0. 46 0. 45 0. 4 0. 43 0. 49 0. 43 0. 41 ∑R EE 19 1. 76 20 9. 27 21 2. 51 22 7. 06 24 9. 86 24 9. 88 19 2. 39 20 8. 03 24 8. 54 23 4. 77 16 3. 65 16 4. 29 24 6. 76 30 1. 07 25 4. 43 16 6. 36 LR EE /H RE E 8. 47 43 8. 60 39 8. 32 06 8. 60 90 8. 81 77 8. 71 16 8. 57 16 8. 54 70 8. 67 46 8. 11 02 6. 69 39 6. 89 48 8. 68 83 9. 51 59 8. 98 16 7. 43 18 G eologia C roatica Wang et al.: Geochemistry and U-Pb ages of the diabases from the Luoji area, western Yunnan, China: implications for the timing of the initial rifting ... 25 similar to the global mean values of CRB, and they have high contents of continental crust enriched elements of Cs (0.39~0.97ppm) and Pb (4.24~7.07ppm), which is different from N-MORB, E-MORB and OIB, so the studied samples belong to CRB (MACDONALD, 2001; MA et al., 2017). The listric-shaped chondrite-normalized REE patterns of the studied samples are also similar to CRB, but are different from N-MORB and OIB (Fig. 5a). In addition, the REE diagram for the samples shows LREEs enrichment with obvious HREEs fractionation so that the REE curve trends are different from N-MORB but similar to OIB and CRB (Fig. 5b). However, the slight increase in Ba, Yb and Lu, and the relative decline of Sr and P are more similar to CRB rather than OIB. A primitive mantle-normalized spider diagram (Fig. 5b) for samples shows obvious enrichment in LILEs (e.g., Figure 5. (a) Chondrite-normalized REE distribution pattern and (b) primitive mantle-normalized spider diagram for the samples (after SUN and MCDONOUGH, 1989). Figure 6. Plots of discrimination diagrams of (a) Zr/4-Nb×2-Y diagram (after MESCHEDE, 1986), (b) Zr-Ti/100-Y×3 diagram (after PEARCE & CANN, 1973), (c) Th-Hf/3- Nb/16 diagram (after WOOD, 1980) and (d) Zr-Zr/Y diagram (after PEARCE & NORRY, 1979). G eo lo gi a C ro at ic a Geologia Croatica 72 / Special Issue26 Rb, Ba, K) and depletion in HFSEs (e.g., U, Nb, Ce), consistent with CRB. The HFSEs (such as Ta, Nb, Ti, Zr, Hf, Th and Y) and HREE in the basic rocks are usually not affected by metamorphism and hydrothermal alteration under the amphibolite phases (PEARCE et al., 1973). HFSEs are an effective tracer for distinguishing dif- ferent tectonic settings and magma source regions (FREY et al., 1978; CAMPBELL & GRIFFITHS, 1993; KHOSHNOODI et al., 2017). According to the tectonic-setting discriminant diagrams of WANG et al. (2002) for basalts, the Th/Ta ratios can be used to distinguish between continent rift and oceanic settings. Luoji diabase samples have lower Hf/Th ratios (1.40~2.82, average 1.75), higher Th/Ta (0.96~2.60, average 1.78) and Ta/Hf (0.24~0.42, average 0.35) ratios than continental within-plate ba- salt (Hf/Th=8, Th/Ta=1.60, Ta/Hf=0.1), but similar to alkaline basalt (Ta/Hf ratios>0.3). All analyzed geochemistry data display within-plate alkaline basalt characteristics, which are consistent with the characteristics of an intracontinental rift basalt. Further, the diabase samples have characteristics of within-plate alkaline basalt and tholeiite from the discrimination diagrams (Fig. 6). In generally, the combination of tholeiite and alkaline basalt series originated from intracontinental rift, initial ocean basin or an ocean island environment (LASSITER & DEPAOLO, 1997; KISS et al., 2016). Furthermore, all data fall into the intraconti- nental and continental margin rift tholeiite-alkaline basaltic field (Fig. 7). A small number of Cuiyi diabase samples, which plot in the mantle plume field, may indicate that the continental rift mag- matism could have been caused by an active mantle plume/an ac- tive mantle mechanism (STOREY & KYLE, 1997). The diabases belong to high-Ti tholeiite to high-K calc-alkaline series with de- pletion in some HFSEs, which is also similar to CRB (MA- HONEY et al., 1995; MACDONALD et al., 2001). All samples fall into the CFB field (Fig. 8), which also shows that the Luoji and Cuiyi diabases are mainly within-plate basalts during the continental rift formation period. 5.2. Magma characteristics of the rocks 5.2.1. Source region In this study, HFSEs and REEs can be good indicators for source regions. In Fig. 9 and Fig. 10a, the samples all originated from a low-degree partial melting of enriched mantle with the participa- tion of a small quantity of crust and/or continental margin mate- rial, which is highly consistent with CRB (CAMPBELL & GRIF- FITHS, 1993; LASSITER & DEPAOLO, 1997). This suggests that the magma source region may have a slight crustal contami- nation (PECCERILLO & TAYLOR, 1976), and all samples fall into the CRB field in the La/Nb vs. La/Ba discrimination dia- grams (Fig. 10b), which also indicates a slight crustal contamina- tion (CUI et al., 2015). High Ce/Yb ratios of the diabases (32.58~33.78) indicate the low-degree of melting and garnets as the main residual phases, and the samples also plot in the garnet stable field (Fig. 11a, CUI et al., 2015; KISS et al., 2016). In the La/Sm vs. Sm/Yb diagram (Fig. 12a), the samples all plot on the upper part of the primitive mantle melting curve with a slight deviation from the melting of the garnet lherzolite field, which may be caused by the low-de- gree melting of the source region and a certain degree of upper crustal contamination (CAMPBELL & GRIFFITHS, 1993; MA- Figure 7. Tectonic setting discrimination diagrams. (a) Ta/Hf vs. Th/Hf diagram (after WANG et al., 2001); (b) Nb/Zr vs. Th/Zr diagram (after SUN et al., 2003); (c) Nb/Zr vs. La/Zr diagram (after WU et al., 2003). Figure 8. Classification diagram of Ti/1000 vs. V for basaltic magma (after SHER- VAIS, 1982). G eologia C roatica Wang et al.: Geochemistry and U-Pb ages of the diabases from the Luoji area, western Yunnan, China: implications for the timing of the initial rifting ... 27 HONEY et al., 1995; JUNG & MASBERG, 1998). Further, the samples plot near the primary garnet lherzolite field and the dis- tribution of the Luoji diabase samples is relatively concentrated (Fig. 12b), which indicate the samples may be the products of partial melting of 2%~3% garnet lherzolite phases (KHOSH- NOODI et al., 2017; MA et al., 2017). Although the distribution of the Cuiyi diabase samples is relatively wide (Fig. 12b), it is still in the vicinity of the primary garnet lherzolite field. The reason why the Ce/Y ratios of the Cuiyi diabase fluctuates greatly may be caused by crustal contamination. Combined with the Hf vs. Hf/Yb diagram (Fig. 11b), it shows that diabases originated in a deep position and were formed by decompression melting of the mantle with a low degree of partial melting and strong differen- tiation of LREE and HREE (especially in the Cuiyi diabase). There is an obvious negative P anomaly in all samples without extreme deficiency in the chondrite-normalized REE distribution pattern (Fig. 5a), which can be combined with three possible rea- sons for negative P anomalies (CAMPBELL & GRIFFITHS, 1993; KISS et al., 2016): a source region deficiency in P; residual apatites in the source region; fractional crystallization of nega- tive P minerals, where apatites remained in the source region re- sulting in less P in samples than in the global CRB-type samples. Figure 9. Source discrimination diagrams. (a) Zr/Y vs. Nb/Y diagram (after SAUNDERS et al., 1997); (b) Nb/Yb vs. Th/Yb diagram (after PEARCE, 1982); (c) La vs. La/Nb diagram (after LI, 1993); (d) Nb vs. Nb/Th diagram (after LI, 1993). Figure 10. (a) Ta/Yb vs. Th/Yb diagram (after PEARCE, 1982); (b) La/Nb vs. La/Ba diagram for crustal contamination (after MILLER et al., 1999). G eo lo gi a C ro at ic a Geologia Croatica 72 / Special Issue28 5.2.2. Assimilation and Contamination The Mg# values of the samples vary from 0.40 to 0.57 and are significantly lower than those of the primary magma (0.68~0.75), which indicates that the rocks had experienced a certain degree of fractional crystallization (FREY et al., 1978). Low contents of MgO and FeO in the samples may indicate the fractional crystal- lization of augite and diopside during the process of magmatic evolution, or the formation of a basic-ultrabasic complex (cumu- lates), which results in a large reduction of Mg (HAWKES- WORTH et al., 1993; JUNG & MASBERG, 1998; KHOSH- NOODI et al., 2017), and then the magmatic differentiation formed the geochemical characteristics of low Mg in the dia- bases. According to the compatibility of elements in different mine- rals, the abundance of elements will change with crystallization. The ratios of elements sensitive to assimilation and contamina- tion are an indicator of assimilation and contamination because of the compositional differences between the crust and the man- tle (CAMBELL & GRIFFITHS, 1993; MACDONALD et al, 2001). The Luoji and Cuiyi diabases show an obvious positive correlation in the Th/Nb vs. Ce/Nb and Ta/Yb vs. Th/Yb dia- grams (Fig. 13a and 13b), indicating that there is a certain assimi- lation and contamination in the samples (HAWKESWORTH et al., 1993; KISS et al., 2016). The Nb/U and Ta/U ratios of the samples are 32.12~76.39 (average 42.10) and 1.48~3.32 (average 2.59), respectively, which are lower than the global relative ho- mogeneous values of MORB and OIB (Nb/U≈47 and Ta/U≈2.7). The Ta/La ratios (0.05~0.07, average 0.062) are slightly higher than the standard value (0.06) of the original mantle (LIANG et al., 2018), which indicate that the diabases are obviously contami- nated by the crust or the source region. It is generally believed that the crustal material has low Nb and high Th characteristics (WANG et al., 2018). The positive correlation between Th and Nb (Fig. 13c), a slight negative correlation between La/Yb and Nb/Ta (Fig. 13d) and a slight negative Nb anomaly in the dia- bases, together indicate that the samples have been affected by crustal contamination (JUNG & MASBERG, 1998). Similarly, the depletion in LILE of Sr in the primitive mantle-normalized spider diagram (Fig. 5b) may be mainly due to the contamination by crustal materials (SZABÓ et al., 2016; KISS et al., 2016). Variation of mantle element contents and Nb/La ratios are an obvious indicator of the process of assimilation, contamina- tion and fractional crystallization (TAYLOR & MCLENNAN et al., 1995). There is a positive correlation between Mg# and Cr, Ni, V, Zr, Sr and Nb/La (Fig. 14), which shows that the fractional crystallization of the diabases are affected by assimilation and contamination (FREY et al., 1978; CAMPBELL & GRIFFITHS, 1993; HAWKESWORTH et al., 1993). Meanwhile, the SiO2 vs. Nb/La diagram (Fig. 15a) indicates that the evolutionary process of the diabases is dominated by fractional crystallization, and Figure 11. (a) The chondrite-normalized (La/Sm)N and (Tb/Yb)N diagram (after WANG et al., 2001; WANG et al., 2002); (b) Hf-Hf/Yb diagram (after WEAVER, 1987). Figure 12. (a) La/Sm vs. Sm/Yb diagram (after LASSITER & DEPAOLO, 1997); (b) Zr/Nb vs. Ce/Y diagram (after GAO et al., 2009). G eologia C roatica Wang et al.: Geochemistry and U-Pb ages of the diabases from the Luoji area, western Yunnan, China: implications for the timing of the initial rifting ... 29 there are signs of contemporaneous assimilation and contamina- tion. In the CaO vs. CaO/Al2O3 diagram (Fig. 15b) the two groups of samples are consistent with a large consumption of Mg in the magma caused by the fractional crystallization with clinopyroxe ne resulting in the low MgO content in the samples. The relative enrichments in Rb, Ba and Th also indicate that magmatic migra- tion has undergone crustal contamination (CUI et al., 2015; KISS et al., 2016). Previous studies (MACDONALD et al., 2001; MA et al., 2017) have shown that strong depletion in Sr of some high- Mg basalts may be affected by contamination or alteration. There- Figure 13. Trace element discrimination diagrams for assimilation and contamination. Figure 14. Plots of selected trace elements with Mg# for discrimination contamination. G eo lo gi a C ro at ic a Geologia Croatica 72 / Special Issue30 fore, negative Sr anomalies and negligible negative Eu anomalies in the samples are not caused by plagioclase fractionation. Combined with the above analysis, the geochemical charac- teristics of major and trace elements in the samples may reflect assimilation and contamination by crustal rocks during the em- placement of the CRB magma. 5.3 Geological significances The timing of the opening of the Ganzi-Litang Ocean basin may have been in the Early Carboniferous, subduction in the Early Triassic and closure in the terminal period of the Late Triassic (WANG et al., 2018). Combined with the recent study, it can be concluded that the study area has experienced three tectonic evo- lutionary stages during the Permian to Triassic period, i.e., an intracontinental rift basin stage during the Early to Middle Per- mian, initial ocean basin stage during the terminal Permian and oceanic crust subduction stage during the Late Permian to Early Triassic (HOU et al., 2001; LIANG et al., 2018). However, the opening time of the Ganzi-Litang Ocean basin is controversial due to the uncertainty of the stretching time and the complexity of the rock exposures. Hence, the accurate development timing especially of the intracontinental rift stage of initial Ganzi-Litang Ocean has never been reported. All the collected data including the zircon U-Pb ages of MORB and OIB, is disadvantageous and incomplete for determining the timing of the opening and the evolutionary processes for the Ganzi-Litang Ocean without the CRB. So a large number of diabase samples were sought in this study to find out the CRB samples. Finally, the Luoji and Cuiyi diabase samples were discovered and have typical CRB charac- teristics discovered through geochemical data analysis. This in- dicates that the crystallization age of the Luoji diabase can accu- rately define the timing of the intracontinental rift stage of the Ganzi-Litang Ocean and is also essential for determining the precise timing of the opening of the Ganzi-Litang Ocean. In view of the zircon structures and Th/U ratios (0.11~5.80), zircons from the Luoji diabase indicate they are of magmatic ori- gin. A Concordia age of the oldest 21 analyses is 1907Ma, which may represent a mantle-derived magmatic event in the western margin of the Yangtze Block in the late Palaeoproterozoic (ZHAO et al., 2010). A set of data in 1245Ma~1028Ma may represent the Grenvillian orogeny and related magmatic events during the Mes- oproterozoic period (LI et al., 2002). Another set of data in 901Ma~757Ma may represent a magmatic event with a span of about 260Ma (1000Ma~740Ma) in the western margin of the Yangtze in the Neoproterozoic (SUN et al., 2009). These zircons would be inherited zircons. For the youngest set of data (303Ma~291Ma), the zircon U-Pb age of the Luoji samples is 293.4±5.4Ma (Early Permian), which should represent the crystal- lization age of the rock. Some zircon U-Pb ages have been ob- tained in the southern part of the Ganzi-Litang suture zone. For example, 341.6±4.9Ma of mid-ocean ridge low-K tholeiite series gabbro in the ophiolite mélange belt, the Baihanchang area, 272.8±1.2Ma for a gabbro-diabase in the Tuguan village and 221.24±0.93Ma of the OFB (floor-ocean ridge basalt)-MORB in Tangyang Country. The age of the Hongshan-Shudu ophiolites determined by Sm-Nd isotope is 230Ma (HOU et al., 2001; YB- GMRRGST, 2003; LIANG et al., 2018). In the middle part of the Ganzi-Litang suture zone, an LA-MC-ICP-MS zircon U-Pb age of the pillow basalt in the Charichadong area is 271±10 Ma. In terms of biostratigraphy, the Early Triassic radiolarians were de- veloped in the Litang Wenquan and Early-Late Triassic radiolari- ans were developed in the Yazui pasture Xigou-Litang Wenquan. In the northern part of the Ganzi-Litang suture zone, the zircon U-Pb age of the gabbro in the N-MORB ophiolite is 292±4Ma. In terms of biostratigraphy, the Early Carboniferous radiolarians in siliceous rocks are developed in the Zhuqing area of northern Ganzi, in addition, Early Carboniferous, Middle Permian, Middle Triassic and Late Triassic radiolarian siliceous rocks occur in the Yushu Xiewu, Dege Country Zhuqinglangduo-Sanchahe and Zhuqing village-Jike (HOU et al., 2001). Based on the zircon U-Pb ages, the Luoji and Cuiyi diabases may belong to the products of the intracontinental rift-initial ocean basin stage. Combining the data of zircon geochronology and biostrati- graphic geochronology from north to south, suggests that there is a trend towards a younging for the timing of the opening of the Ganzi-Litang Ocean from the north to the south, which may be related to the evolution of the Ganzi-Litang Ocean. The U-Pb age of the intracontinental rift-type diabase in the study area is 293.4±5.4Ma, which is analogous with the zircon age of the N-MORB ophiolite in the Ganzi area (LIANG et al., 2018). This also shows that the opening of the Ganzi-Litang Ocean in the southern section may have lagged behind that in the northern section. Therefore, the comprehensive analysis shows that the study area started to enter into an intracontinental rift stage of initializing the Ganzi-Litang Ocean at 293.4±5.4Ma (Early Permian) and formed an enriched mantle parent magma with CRB affinity. Then the parent magma formed the Luoji dia- base in the intracontinental rift environment through crystalliza- tion and differentiation, assimilation and contamination and as- cending emplacement. Figure 15. Discrimination diagrams for fractional crystallization and assimilation-contamination. (a) SiO2 vs. Nb/La diagram; (b) CaO vs. CaO/Al2O3 diagram. G eologia C roatica Wang et al.: Geochemistry and U-Pb ages of the diabases from the Luoji area, western Yunnan, China: implications for the timing of the initial rifting ... 31 6. CONCLUSION 1. The Luoji and Cuiyi diabases are high-K alkaline to - tholeiite with low Si, high Ti, K, Fe and Zr characteristics, which are similar to CRB but different from N-MORB and OIB. 2. The diabases were formed by low-degree partial melting of an enriched mantle source consisting of garnet lherzolite with slight crustal contamination. 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