Geochemistry of lamprophyres from western Thua Thien Hue

Kỷ yếu Hội nghị: Nghiên cứu cơ bản trong “Khoa học Trái đất và Môi trường” - Những kết quả nghiên cứu mới DOI: 10.15625/vap.2019.00077 11 GEOCHEMISTRY OF LAMPROPHYRES FROM WESTERN THUA THIEN HUE Nguyen Thi Thuy, Le Hai Nghia, Hoang Hoa Tham Hue University of Sciences, Hue University, No. 77 Nguyen Hue st., Hue city, Vietnam Email: ntthuykh@hueuni.edu.vn ABSTRACT Three lamprophyre bodies at Khe Phen and Xuan Long (western Thua Thien Hue) have been mineralogically and geochemically investigated. The rocks are exposed as narrow melanocratic dikes crossing the Ba Na granites and mainly composed of hornblende, plagioclase and quartz. Whole-rock major element compositions are characterized by low SiO2 (49.24-54.23 wt.%) and high calc-alkaline contents (CaO: 5.03-9.17 wt.%, Na2O + K2O: 2.82-6.63 wt.%), defining them into calc-alkaline lamprophyre category. Rare earth element (REE) concentrations are relatively as low as 71.9-238.0 ppm. High (La/Lu)cn ratios (4.6-44.6) associated with negative Ta, Nb and Ti anomalies of the lamprophyres might be due to destructive plate-margin magmatism and probably indicate an assimilation of subducted sediment slabs. The lamprophyres must be more powerful in deciphering magmatic activities and tectonic setting during Permo-Triassic stage in North margin of the Kontum massif in case their isotopic compositions are further collected. Keywords: Geochemistry, lamprophyre, Thua Thien Hue. 1. INTRODUCTION Lamprophyres in Vietnam territory have been seriously noticed in recent decades, mainly including those in South Central Vietnam [2, 3, 8]. The studies focus not only on mineralogical and geochemical characteristics, but also on isotopic composition, by which the ages and tectonic implications as well as relationship between the lamprophyres and adjacent magmatisms in the areas have been deciphered progressively. In this paper, we present for the first time geochemical data of the lamprophyre dikes cropping out at Khe Phen and Xuan Long granite quarries (western Thua Thien Hue Province). Geologically, the dikes are relatively narrow as about 0.5-0.7 m (at Khe Phen) to 1.5-2.0 m (at Xuan Long) in width, crossing the Ba Na granites (Fig. 1a, b). The rocks have a massive poikilitic texture and are green to black in hand specimens (Fig. 2a, b). Mineral assemblage is characterized by hornblende (50-55 vol.%), plagioclase (33-40 vol.%) and quartz (3-5 vol.%); calcite, chlorite are secondary minerals; apatite, epidote and opaque are rare (Fig. 3a, b). 2. METHODOLOGY Major element contents of whole rocks were conducted by a S4 Pioneer X-ray fluorescence spectrometer installed at the Institute of Geology. Rare earth and trace element compositions were determined using a Varian’s Ultra Mass-700 Inductively coupled plasma mass spectrometer situated at the Institute of Geology, analyzed data were corrected by using GSJ JB-1a standard. 3. RESULTS AND DISCUSSION 3.1. Whole-rock major geochemistry Major contents for whole-rock of the lamprophyres are shown in Table 1. The lamprophyres are typical of low SiO2 (49.24-54.23 wt.%) and high calc-alkaline contents (CaO: 5.03-9.17 wt.%, Na2O + K2O: 2.82-6.63 wt.%). Aluminum saturation index values are greater than 1 excepting the Hồ Chí Minh, tháng 11 năm 2019 12 sample KPLP01 (ASI: 0.97). K2O/Na2O ratios vary from 0.56 to 0.86. These features of the rocks define them into calc-alkaline lamprophyre branch [7]. In the discriminating diagrams for the whole-rock compositional fields of lamprophyres (K2O vs. SiO2 and Na2O + K2O vs. SiO2) given by Rock (1984) [6], the analyzed samples were plotted into medium- to high-K calc-alkaline lamprophyre field. 3.2. Whole-rock trace element geochemistry Trace element concentrations of the lamprophyres are given in Table 2. The lamprophyres show lower total-REE concentrations ranging from 71.9 ppm to 238.0 ppm in comparison with calc-alkaline lamprophyre worldwide (531.1 ppm in average, Rock 1987) as well as with Ba To, Bong Son and Dak Long lamprophyres (at the margins of the Kontum massif [3]). However, the normalized abundance patterns of REEs by chondrite show a common trend of normal lamprophyre worldwide with (La/Lu)cn from 4.6 to 44.6 (Fig. 4). In the primitive mantle-normalized element abundance patterns, slightly negative Eu and Nb anomalies coupled with positive Rb, Th, Zr, Hf, Gd anomalies are present for all samples, which are commonly found in calc-alkaline lamprophyres, but not in alkaline lamprophyres of ultramafic lamprophyres. Particularly, negative anomalies of Ta, Nb and Ti in the spidergram (Fig. 4) might be attributed to destructive plate- margin magmatism and have been interpreted as indicating assimilation of subducted sediment slabs ([7] and references there-in). 3.3. Implications of studying the lamprophyres Studies on lamprophyres in Vietnam have demonstrated their implications in revealing tectonic settings and magmatic activities in the area, particularly in Mesozoic era (eg. [2]). Appearance of lamprophyres in western Thua Thien Hue, whose mineralogical and geochemical features are very much alike those recorded at Ba To, Bong Son, Dak Long (at the margins of the Kontum massif) puts forward a large-scale study on the lamprophyres and their tectonic significance, especially in Permo - Triassic stage. Interestingly, on the geological map scale 1: 200.000, Huong Hoa - Hue - Da Nang sheet, the Ba Na granites have been determined to be compositionally acidic and latest intrusive rocks (K-Ar age of 130-40 Ma, corresponding Cretaceous - Paleogene [1]). Nevertheless, the lamprophyres’ occurrences strongly approve that the Ba Na granites had been formed at the age earlier than Cretaceous - Paleogene (at about 245 Ma, corresponding to Early Triassic [5]). 4. CONCLUSION Field surveys, mineralogical and geochemical data of the lamprophyres at Khe Phen and Xuan Long (western Thua Thien Hue) could lead to following remarks: (1) The mineral assemblages, whole-rock major and trace chemical compositions of the dikes define them as calc- alkaline lamprophyres; (2) High (La/Lu)cn ratios (4.6 - 44.6) associated with negative Ta, Nb and Ti anomalies of the lamprophyres might be due to destructive plate-margin magmatism and probably indicate an assimilation of subducted sediment slabs; (3) Studying the lamprophyres could provide informative clues in interpreting magmatic activities and tectonic setting during Permo-Triassic stage in North margin of the Kontum massif. REFERENCES [1]. Phan Luu Anh, Tran Trong Hoa, Vladimirov A. G., Tran Tuan Anh, 1995. Formation conditions of the Hai Van, Ba Na-typed granitoids using updated data of trace elements and isotopes. Journal of Earth Sciences, Vol. 17(4): 151-155. Hanoi. [2]. Tran Tuan Anh, Tran Trong Hoa, Izokh A.E., Borisenko A.E. and Travin A.V., 2006. Age constraints on petrogenesis of lamprophyre form South Central Vietnam. Journal of Geology, 27(B): 23-29. Kỷ yếu Hội nghị: Nghiên cứu cơ bản trong “Khoa học Trái đất và Môi trường” 13 [3]. Tran Tuan Anh, Tran Trong Hoa, Ngo Thi Phuong, Pham Thi Dung and Mai Kim Vinh, 2008.Geochemical significance of the Triassic lamprophyre at the margins of the Kon Tum block.Vietnam Journal of Earth Sciences, 30(3): 210-224. In Vietnamese with an English abstract. [4]. McDonough W.F. and Sun S.S., 1995. The composition of the earth. Chem.Geol., 120: 223-253. [5]. Nguyen Trung Minh, 2005. Determining emplacement age of the Ba Na block corresponding to the Indochina tectonic stage using U-Pb isotopic analysis.Journal of Geology, A287. Hanoi. [6]. Rock N.M.S., 1984. The nature and origin of calc-alkaline lamporphyres: minettes, vogesites, kersantites and spessartites. Trans. R. Soc. Edinb., 74: 193-227. [7]. Rock N.M.S., 1987. The nature and origin of lamprophyres: An overview. Geological Society, London, Speccial Publications, 30: 191-226. [8]. Mai Kim Vinh, 2002. Geological features, petrography, geochemistry of high alkaline lamprophyre in Ba Bich (Quang Ngai) and An Quang (Binh Dinh) areas. Geology and Environment of Vietnam, 63- 73.In Vietnamese. a. b. Figure 1. Occurrence of the lamprophyre dikes at Khe Phen (a) and Xuan Long (b). a. b. Figure 2. Hand specimens of the lamprophyres: a- Khe Phen, b- Xuan Long. a. b. Hồ Chí Minh, tháng 11 năm 2019 14 Figure 3. Microphotographs of the lamprophyres: a- Nicol +, b- Nicol - Symbols: q- quartz, pl- plagioclase, or- orthoclase, bt- biotite, cx- calcite, hb- hornblende, qu- opaque. Table 1. Whole-rock major element compositions of the lamprophyres (wt.%) Sample XLLP01 XLLP03T XLLP03N KPLP01 Sample XLLP01 XLLP03T XLLP03N KPLP01 SiO2 54.23 50.41 52.15 49.24 Na2O 3.56 2.04 3.22 1.81 TiO2 0.95 1.09 1.15 0.85 K2O 3.07 1.53 1.86 1.01 Al2O3 12.88 14.36 12.96 12.17 P2O5 0.34 0.27 0.54 0.15 TFe2O3 11.7 14.97 13.78 15.41 SO3 0.19 0.18 0.23 0.07 MnO 0.15 0.16 0.13 0.2 LOI 1.22 0.36 - - MgO 6.5 7.06 7.18 9.17 Total 99.8 99.8 99.6 99.8 CaO 5.03 7.35 6.43 9.74 Na+K 6.63 3.57 5.08 2.82 ASI 1.1 1.32 1.13 0.97 Table 2. Whole-rock trace element compositions of the lamprophyres (ppm) Sample XLLP01 XLLP03T XLLP03N KPLP01 Sample XLLP01 XLLP03T XLLP03N KPLP01 Rb 180.84 77.74 60.44 83.39 Eu 2.32 1.44 2.11 0.92 Ba 1065.19 468.97 860.01 247.26 Gd 7.7 4.98 6.69 3.56 Th 19.62 17.25 85.33 3.31 Tb 0.72 0.64 0.75 0.55 U 2.92 10.93 10.22 4.79 Dy 3.21 3.65 3.26 3.52 Nb 26.05 19.71 55.47 16.27 Y 18.24 20.57 17.08 22.44 Ta 1.94 3.18 10.46 2.09 Ho 0.55 0.65 0.58 0.66 La 57.52 20.97 53.73 12.35 Er 1.62 1.98 1.65 2.23 Ce 102.05 42.03 99.87 25.45 Tm 0.23 0.28 0.3 0.32 Pr 11.89 5.35 11.88 3.38 Yb 1.3 1.7 1.28 1.97 Sr 1060.42 537.12 1352.19 343.48 Lu 0.2 0.24 0.29 0.28 Nd 41.86 21.26 43.28 13.4 La/Lu 287.6 87.4 185.3 44.1 Sm 6.84 4.54 7.41 3.27 REE 238.0 109.7 233.1 71.9 Zr 734.71 334.72 473.06 381.51 Total 3369.1 1630.6 3167.4 1186.7 Hf 21.15 30.69 10.04 10.28 Figure 4. Normalized abundance patterns of rare earth elements by chondrite and of trace elements by primitive mantle for the lamprophyres from Khe Phen and Xuan Long, western Thua Thien Hue. Chondrite and primitive mantle data from McDonough and Sun (1995) [4], calc-alkaline lamprophyre worldwide from Rock (1987) [7]. 1 10 100 1000 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu XLLP01 XLLP03T XLLP03N KPLP01 Calc-alkaline lamprophyre worldwide S am p le /C h o n d ri te 1 10 100 1000 10000 R b B a T h U T a N b K L a C e P r S r N d S m Z r H f E u T i G d T b D y Y H o E r T m Y b L u S a m p le /P ri m it iv e M a n tl e