타이와 베트남에서 수집된 텍타이트의 화학조성과 지구화학적 의의

이승구,Tsuyoshi Tanaka,Yoshihiro Asahara,Masayo Minami 한국암석학회 2017 암석학회지 Vol.26 No.3

We determined chemical compositions like abundance of major and trace elements, Sr and Nd isotope compositions for two tektites from the Thailand and Vietnam. Their chemical compositions are similar to each other, and seem to be similar to those of PAAS (Post Archean Australian Shale) rather than upper continental crust. In particular, primitive mantle-normalized spider diagrams and chondrite-normalized REE patterns for two tektites are the same, suggesting that they might be derived from the same source material. The 87Sr/86Sr and 143Nd/144Nd ratios from Thailand tektite are 0.718870±0.000008 (2σm) and 0.512024±0.000012 (2σm), respectively, and those from Vietnam are 0.717022±0.000008 (2σm) and 0.511986±0.000013 (2σm), respectively. The 87Sr/86Sr and 143Nd/144Nd ratios from Thailand tektite are slightly enriched than those of Vietnam tektite. 87Sr/86Sr ratios from the Vietnam and Thai tektites were plotted on the range of Australasian tektites reported previously. 143Nd/144Nd ratio of Vietnam tektite from this study was lower than the range of 143Nd/144Nd ratio from the Australasian tektite reported previously whereas that of Thai tektite was included in the range of 143Nd/144Nd ratio from the Australasian tektite. The geochemical characteristics from two tektites in this study indicate that they may be derived from the very similar source materials. 타이와 베트남산 텍타이트의 주원소 조성, 희토류원소를 포함하는 미량원소 조성, Sr과 Nd의 동위원소 화학조성을 측정하여 상호간의 연관성을 비교하였다. 두 텍타이트의 주원소 조성은 서로 유사하며, 상부지각의 화학조성보다는 PAAS(Post Archean Australian Shale)의 화학조성에 더 유사하다. 거미도와 희토류원소의 분포도는 서로 간에 일치하는 특성을 보여주며, Eu의 부(−)의 이상과 더불어 경희토류가 부화되고 중희토류가 결핍되어 있는 특성 또한 PASS의 희토류원소 분포도와 유사하다. 타이산 텍타이트의 87Sr/86Sr과 143Nd/144Nd 비는 각각 0.718870±0.000008 (2σm), 0.512024±0.000012 (2σm)이고 베트남산 텍타이트의 87Sr/86Sr과 143Nd/144Nd 비는 0.717022±0.000008 (2σm). 0.511986±0.000013 (2σm)으로 타이산 텍타이트가 베트남산 텍타이트보다 더 부화된 동위원소 값을 갖고 있다. 그리고 현재까지 알려진 오스트레일리아와 동아시아에서의 텍타이트의 87Sr/86Sr과 143Nd/144Nd 비와 비교했을 때, 이 연구에서의 두 지역 텍타이트의 87Sr/86Sr비 값은 모두 기존에 알려져 있는 Australasian 텍타이트에서의 값들의 범위에 포함된다. 143Nd/144Nd 비의 경우 타이산 텍타이트는 현재 알려져있는 Australasian 텍타이트의 143Nd/144Nd비 값 범위에 들어가는 반면에, 베트남산 텍타이트의 143Nd/144Nd비 값은 현재까지 알려진 143Nd/144Nd 비보다 낮았다. 이 연구에서의 두 텍타이트의 지구화학적 특성의 유사성은 두 텍타이트가 거의 동일한 기원물질로부터 유래되었을 가능성을 지시해준다.

백령도에 분포하는 알칼리 현무암과 맨틀 포획암의 Nd-Sr과 영족기체 동위원소 조성

김규한(Kim Kyu Han),長尾敬介(Nagao Keisuke),장형숙(Jang Hyung Sook),角野浩史(Sumino Hirochika),정정인(Chung Jung In) 대한자원환경지질학회 2002 자원환경지질 Vol.35 No.6

백령도에서 산출되는 알칼리 현무암과 현무암내의 맨틀포획암에 대하여 REE와 Nd-Sr, 영족기체의 동위원소비를 분석하여 초염기성 포획암과 알칼리 현무암질 마그마의 기원을 해석하였다. 연구결과를 요약하면 다음과 같다. (1) 스피넬-러조라이트 맨틀 포획암을 다량 함유한 알칼리 현무암은 주로 현무암-뮤저라이트(mugearite)와 현무암질 안산암으로 구성되어 있다. (2)높은 HREE특징을 가지는 알칼리 현무암의 REE패턴은 OIB패턴과 유사하다. 그리고 상이한 시료들 간에 비교적 균일한 REE패턴은 알칼리 현무암질 마그마가 동일 기원물질에서 유래하였음을 시준하고 있다. (3)현무암의 Nd-Sr동위원소비는 알칼리 현무암질 마그마가 지각물질의 혼입영향이 없는 결핍맨틀 기원임을 가르키고 있다. (4)포획암중 감람석의 헬릅동위원소비(3He/4He)는 5.0±1.1Ra-6.7±1.3Ra로 MORB 값(ca. 8.0Ra) 보다 낮으며 대륙지각 하부 리소스피어 맨틀의 특성을 나타내고 있다. 그러나 시료 OL-7 감람석의 높은 3He/4He(16.8±3.1Ra) 값은 분출후 우주선 기원 3He의 혼입으로 추정된다. 감람석의 아르곤 동위원소비(40Ar/36Ar=300∼500)는 MORB값(10,000∼30,000)보다 훨씬 낮은 대기 아르곤의 동위원소비(295.5)에 가깝다. 이는 백령도 포획암의 감람석이 마그마 분출후 느린 탈가스 과정에 대기의 혼염이 있었음을 암시하고 있다 The rare earth elements (REE) and Nd, Sr and noble gas isotopic compositions (3He/4He, 40Ar/36Ar) for the Quaternary alkali basaltic rocks and mantle xenoliths in the basaltic rocks from the Baegryongdo were investigated to decipher the origin of alkali basaltic magma and xenolith beneath the Sino-Korean craton. Analytical results are summarized as follows; (1) The alkali volcanic rocks with voluminous xenoliths which are represented by the Mg-olivine and clinopyroxene dominant spinel-lherzolite in the Baegryongdo consist mainly of the basalt-mugearite and basaltic andesite. (2) The REE pattern of alkali basaltic rocks characterized by high HREE is similar to that of oceanic island basalt (OIB). Relatively concordant REE patterns of the basaltic rocks suggest that the alkali basaltic magma be formed by the identical source materials. (3) The Nd-Sr isotopic data of the alkali basaltic rocks suggest that the alkali basaltic magma be originated from the depleted mantle source with a little contamination of the continental crustal materials. (4) The 3He/4He ratios in olivines of xenoliths ranging from 5.0±l.1Ra to 6.7±1.3Ra are lower than that of MORB (ca. 8.0Ra). It suggest that the xenolith be derived from the subcontinental lithospheric mantle. However, the high 3He/4He value of 16.8±3.1Ra at 1800℃ fraction (sample no OL-7) might be resulted from the post-eruptive cosmogenic 3He. The 40Ar/36/Ar ratios in olivines of mantle xenoliths are comparable to that of atmospheric argon, and are much lower than that of the MORB type mantle. These facts can lead to conclusion that the olivine of the xenolith in the Baegryongdo is affected by the post-eruptive atmospheric contamination during the slow degassing process.

Origin of adakite-like plutons in southern Korea

Oh, J.I.,Choi, S.H.,Yi, K. Universitetsforlaget ; Elsevier Science Ltd 2016 Lithos Vol.262 No.-

<P>We present Sr-Nd-Pb isotopic compositions for two adakite-like intrusions in southern Korea (Jindong and Bongnae), including major and trace element concentrations, and the sensitive high-resolution ion microprobe (SHRIMP) zircon U-Pb age. Our aim is to constrain the origin and tectonomagmatic processes that gave rise to the plutons. A SHRIMP U-Pb zircon age of 88.7 +/- 0.7 Ma was obtained from a Jindong granodiorite sample. The Jindong plutons belong to the medium-K calc-alkaline series, and have an overall enrichment in large ion lithophile elements (LILE), such as K, Rb, Ba, and Pb, and a relative depletion in high field strength elements, such as Nb, Ta, and Ti, compared with the neighboring elements in the primitive mantle-normalized multi trace element variation diagram. It was found that (Sr-87/Sr-86); = 0.70475-0.70596, (Nd-143/Nd-144)(i) = 0.512547-0.512604, [(epsilon(Nd))(i) = +0.4 to 22], and (Pb-206/Pb-204)(i) = 18.19-18.37, which fall within the field of the arc-type Cretaceous to Tertiary Bulguksa intrusive rocks in the Sr-Nd and Pb-Pb isotopic correlation diagrams. The elevated Sr/Y (17-40), but low La/Yb (3.7-8.8) ratios of the Jindong plutons, together with their coherent geochemical trends and U-shaped rare earth element (REE) patterns, indicate that they were not formed by slab melting, but by amphibole-dominated fractional crystallization of the Bulguksa-like arc magma. A SHRIMP U-Pb zircon age of 226.5 +/- 2.5 Ma was obtained from a Bongnae tonalite sample. The Bongnae plutons belong to the shoshonitic series. Their trace element patterns resemble the Jindong samples, but are characterized by significantly negative U anomalies in mafic rocks. They also have elevated MgO, Ni, Co, Cr, Rb, Ba, Sr, and low Al2O3 and Na2O contents at a given SiO2 concentration compared with the Jindong plutons, and are typified by highly radiogenic Sr and unradiogenic Nd and Pb isotopic compositions: [(Sr-87/Sr-86); = 0.71096-0.71290, (Nd-143/Nd-144)(i) = 0.511641-0.511681, (epsilon(Nd))(i) = -13.0 to -13.8, and (Pb-206/Pb-204)(i) = 17.53-17.45]. The Bongnae plutons have adakite-like elevated Sr/Y (36-56) and La/Yb (12.9-42.7) ratios, but are highly potassic compared with slab melts. The petrogenesis of the C-type Bongnae adakites is consistent with formation by the partial melting of two components (a lower crust and metasomatized lithospheric mantle) in a post-collisional tectonic setting. Asthenospheric upwelling induced by slab breakoff after continental collision between the north and south China blocks may be a heat source for melting of the continental lithosphere. The Paleo-Tethys subduction before continental collision in this area or lower crustal delamination and subsequent low fractions of melting may have induced metasomatic overprinting in LILE in the overlying lithospheric mantle. (C) 2016 Elsevier B.V. All rights reserved.</P>

제주도 주변 대륙붕 퇴적물의 REE와 Sr-Nd 동위원소 조성

김태정,윤정수 한국지구과학회 2012 한국지구과학회지 Vol.33 No.6

REE, major and trace elements, and Sr-Nd isotopic ratios of surface sediments around Jeju Island were analyzed for identifying the origin of the sediments. The Chemical Index of Alteration (CIA) between 44.2 to 68.9 (av. 59.4) shows a similarity with the Huanghe sediment. The most sediments found within the study areas show a very similar chondrite-normalized REE pattern that has enriched LREE (La(N)/Sm(N)>3) and small negative Eu anomaly, typically of average shales. The UCC-nornalized REE patterns of the southwestern offshore sediment samples show a very similar pattem with the Changjiang sediment with enriched in most REE and more convex REE pattern than those of the Huanghe and Keum rivers sediments, which indicates that the Changjiang River’s suspended sediments have been transported into the western part of Jeju Island. The 87Sr/86Sr isotopic ratios vs εNd(0) values were thus used as a tracer to discriminate the provenance of sediments in the study area. Based on the discriminated diagram, it clearly showed that most sediments in the western and northwestern part were closely plotted with sediments of the Huanghe River. However, the sediments in the southwestern part near the Changjianf estuary were closely plotted with submerged delta sediments of the Changjiang River. In contrast, the sediment samples of the northeastern part showed discriminative figures from those of the Chinese rivers. It suggests that sediments around Jeju Island must be originated from diverse sources. 제주도 주변 대륙붕해역에 분포하는 퇴적물의 기원지를 해석하기 위해 표층퇴적물의 희토류원소(REE), 주성분 및 미량원소, Sr-Nd 동위원소비를 분석하였다. 퇴적물의 화학원소에 근거한 변질지수(CIA)는 44.2-68.9(av.59.4)의 범위를 보이며 황하강퇴적물과 유사하였다. 연구지역 퇴적물을 콘드라이트로 표준화한 REE 패턴에서 대부분 LREE가 부화된(La(N)/Sm(N)>3) 작은 Eu 부(-) 이상을 가지는 전형적인 셰일의 희토류원소의 패턴을 나타낸다. UCC로 표준화한 REE패턴에서 연구지역 남서쪽 외해지역의 퇴적물 시료들은 황하나 금강기원 퇴적물보다 양자강퇴적물과 유사한 높은 희토류원소함량과 위로 볼록한 REE 패턴을 보여 양자강기원 부유물질이 제주도 서쪽해역까지 운반되고 있음을 의미한다. 87Sr/86Sr 동위원소비와 εNd(0)의 구분지수는 연구지역 퇴적물의 기원지를 밝히는데 지시자로 제시할 수 있었다. 연구지역의 서쪽과 북서쪽에 분포하는 대부분의 퇴적물은 황하강 퇴적물 주위에 밀집 분포하는 특징을 보이고, 양자강 하구역과 가까운 남서쪽지역의 퇴적물들은 양자강 수중삼각주 퇴적물과 유사성을 보였다. 반면 제주도 북동쪽지역의 퇴적물은 중국의 강기원 퇴적물과는 다른 지역에 위치하는 특징을 보여, 따라서 제주도 주변해역은 복합기원 퇴적물이 집적되고 있음을 의미한다.

Trace element and isotopic evidence for temporal changes of the mantle sources in the South Shetland Islands, Antarctica

Lee, Mi Jung,Lee, Jong Ik,Choe, Won Hie,Park, Chung-Hwa Geochemical Society of Japan] 2008 Geochemical journal Vol.42 No.2

We present Sr-Nd-Pb isotope data from the Paleocene-Eocene volcanic rocks in King George Island, South Shetland Islands, West Antarctica. The initial isotopic ratios of the analyzed samples display limited variations: <SUP>143</SUP>Nd/<SUP>144</SUP>Nd, 0.512790 to 0.512905 ( εNd, +4.2 to +6.5); <SUP>87</SUP>Sr/<SUP>86</SUP>Sr, 0.703342 to 0.703877 (εSr, -15.6 to -8.0); <SUP>204</SUP>Pb/<SUP>206</SUP>Pb, 18.48 to 18.64; <SUP>207</SUP>Pb/<SUP>204</SUP>Pb, 15.50 to 15.64; <SUP>208</SUP>Pb/<SUP>204</SUP>Pb, 37.99 to 38.41. We interpret these data in combination with previously published trace and isotope data for Meso-Cenozoic volcanic rocks in the South Shetland Islands to gain a better understanding of the geochemical evolution of the mantle source region. The studied rocks are from four volcanic islands and range in age from 143 to 44 Ma. They have high abundances of large ion lithophile elements and light rare earth elements relative to high field strength elements, consistent with products of subduction related magmatism. The systematic inter-island variations are recognized from a comprehensive examination of the trace elements and isotopic compositions. The degree of enrichment of Sr-Nd-Pb isotopic compositions decreases towards younger samples, while the ratios of fluid-mobile elements/ HFSE (Sr/Yb, Pb/Yb and U/Yb) gradually increase. The previous studies on these volcanic rocks concluded that the compositional variations of the South Shetland Islands volcanic suites were mainly controlled by two component mixing between altered MORB and Pacific sediments. However, we here propose that the compositional trends observed in the volcanic rocks of the South Shetland Islands can be created from the addition of a relatively constant subduction component to temporally varying heterogeneous mantle sources. The higher radiogenic Pb and Sr isotopes and lower <SUP>143</SUP>Nd/ <SUP>144</SUP>Nd ratios of the older volcanic rocks from Greenwich and Livingston islands compared to younger rocks can be explained by the significant influence of enriched previously metasomatized mantle material rather than fluids or sediment melts from the subducting slab. In contrast, the geochemical nature of the youngest King George Island volcanic rocks suggests a relatively large contribution of a slab-derived fluid component to the magma generation, but a minor role of the enriched component.

Geochemistry of anorthositic xenolith and host tholeiite basalt from Jeju Island, South Korea

백설,최성희,이승구,이승렬,이효민 한국지질과학협의회 2014 Geosciences Journal Vol.18 No.2

Volcanic rocks on Jeju Island consist mainly ofintraplate alkaline lavas with subordinate intercalated tholeiitebasaltic rocks. The alkaline suite can be divided into high-Al alkali(HAA) and low-Al alkali (LAA) sub-suites. We determined the87Sr/86Sr ratios of anorthositic xenolith plagioclases and the Sr-Nd-Pb isotope compositions of the host tholeiite basalt (Taeheungribasalt). The anorthositic xenoliths are considered to representbuoyancy-induced floating cumulates from previous tholeiitemagmatism on Jeju Island. The 87Sr/86Sr ratios of the xenolithplagioclases are highly elevated compared with those of the Jejutholeiite lavas, including the host basalt. The 87Sr/86Sr ratios ofthe Jeju volcanic rocks increase from the LAA through the HAAto the tholeiite suite. The tholeiite suite generally exhibits lessradiogenic 143Nd/144Nd ratios than the alkaline suite. In Pb-Pbspace, the tholeiite suite has, in general, elevated 207Pb/204Pbratios compared with those of the alkaline suite at a given 206Pb/204Pb ratio. In contrast to the 87Sr/86Sr ratios, the two alkalinesub-suites (HAA and LAA) display indistinguishable Nd and Pbisotope compositions. Notably, some of the tholeiite and HAAsuites exhibit distinctive positive anomalies in Eu and Sr in thetrace element distribution pattern and the 87Sr/86Sr ratios increaseas a function of the degree of Sr or Eu enrichment. These observations,along with the relatively fractionated nature of the tholeiiteand especially the HAA suites, indicate that the suites might beproducts of assimilation of plagioclase-rich cumulates, possibly atlower crustal depth, during parent magma evolution. Jeju volcanicrocks exhibit a mixing line between Indian mid-ocean ridgebasalt-like depleted mantle and enriched mantle type 2 (EM2)components in Sr-Nd and Pb-Pb isotope correlation diagrams. The enriched component, the source of which may have been theasthenosphere, is supposed to have been higher in the Jejutholeiite magmatism than in the alkaline magmatism.

Sr-Nd-Pb isotopic compositions of submarine alkali basalts recovered from the South Korea Plateau, East Sea

이미정,이종익,권성택,추미경,정갑식,조진형,김성렬 한국지질과학협의회 2011 Geosciences Journal Vol.15 No.2

We present geochemical and isotope results (Sr, Nd, Pb) on submarine basalts dredged from the southeastern margin of the South Korea Plateau. The present data are the first geochemical result obtained on submarine basaltic lavas recovered from the Korea Plateau, which is considered to be one of rifted remnants of continent in the East Sea (Sea of Japan) and partially segmented from the Korean Peninsula. The South Korea Plateau basalts (SKPB) show alkali affinities and are characterized by Ocean Island Basalts (OIB)-like trace element compositions relatively enriched in Ba, Nb and Pb. They display relatively flatter [(Ce/Yb)n = 4.49~5.46] and less fractionated HREE [(Dy/Yb)n = 1.11~1.29] patterns compared to average OIB, implying that the SKPB were derived from a relatively shallower mantle depth with a larger degree of partial melting. The Sr, Nd and Pb isotopic compositions of the SKPB display a range of values for ^(87)Sr/^(86)Sr (0.70378 to 0.70409), ^(143)Nd/^(144)Nd (0.512846 to 0.512860) and ^(206)Pb/^(204)Pb (17.88–18.44). They show a considerably high range of ^(207)Pb/^(204)Pb (15.48–15.62) and ^(208)Pb/^(204)Pb (37.86–38.46) values, indicating the involvement of a DUPAL-like MORB source. According to our two-stage magma mixing model, the isotopic compositions of SKPB can be explained by magma source mixing by the addition of an EMII-like mantle component (up to 2%) to the DUPAL-like MORB source embedded with endogenous EMI-like material (about 2%).

Geochemical constraints on the spatial distribution of recycled oceanic crust in the mantle source of late Cenozoic basalts, Vietnam

Hoang, Thi Hong Anh,Choi, Sung Hi,Yu, Yongjae,Pham, Trung Hieu,Nguyen, Kim Hoang,Ryu, Jong-Sik Elsevier 2018 Lithos Vol.296 No.-

<P><B>Abstract</B></P> <P>This study presents a comprehensive analysis of the major and trace element, mineral, and Sr, Nd, Pb and Mg isotopic compositions of late Cenozoic intraplate basaltic rocks from central and southern Vietnam. The Sr, Nd, and Pb isotopic compositions of these basalts define a tight linear array between Indian mid-ocean-ridge basalt (MORB)-like mantle and enriched mantle type 2 (EM2) components. These basaltic rocks contain low concentrations of CaO (6.4–9.7wt%) and have high Fe/Mn ratios (>60) and FeO/CaO–3MgO/SiO<SUB>2</SUB> values (>0.54), similar to partial melts derived from pyroxenite/eclogite sources. This similarity is also supported by the composition of olivine within these samples, which contains low concentration of Ca and high concentrations of Ni, and shows high Fe/Mn ratios. The basaltic rocks have elevated Dy/Yb ratios that fall within the range of melts derived from garnet lherzolite material, although their Yb contents are much higher than those of modeled melts derived from only garnet lherzolite material and instead plot near the modeled composition of eclogite-derived melts. The Vietnamese basaltic rocks have lighter δ<SUP>26</SUP>Mg values (−0.38±0.06‰) than is expected for the normal mantle (−0.25±0.07‰), and these values decrease with decreasing Hf/Hf* and Ti/Ti* ratios, indicating that these basalts were derived from a source containing carbonate material. On primitive mantle-normalized multi-element variation diagrams, the central Vietnamese basalts are characterized by positive Sr, Eu, and Ba anomalies. These basalts also plot within the pelagic sediment field in PbPb isotopic space. This suggests that the mantle source of the basalts contained both garnet peridotite and recycled oceanic crust. A systematic analysis of variations in geochemical composition in basalts from southern to central Vietnam indicates that the recycled oceanic crust (possibly the paleo-Pacific slab) source material contains varying proportions of gabbro, basalt, and sediment. The basalts from south-central Vietnam (12°N–14°N) may be dominated by the lowest portion of the residual slab that contains rutile-bearing plagioclase-rich gabbroic eclogite, whereas the uppermost portion of the recycled slab, including sediment and basaltic material with small amounts of gabbro, may be a major constituent of the source for the basalts within the central region of Vietnam (14°N–16°N). Finally, the southern region (10°N–12°N) contains basalts sourced mainly from recycled upper oceanic crust that is basalt-rich and contains little or no sediment.</P> <P><B>Highlights</B></P> <P> <UL> <LI> The Vietnam basalts have Sr-Nd-Pb isotopic compositions defining a DMM-EM2 array. </LI> <LI> The mantle source is composed of garnet peridotite and recycled oceanic crust. </LI> <LI> Southern to central Vietnam can be divided into three geochemical mantle domains. </LI> <LI> Their light Mg isotopic compositions are likely inherited from recycled carbonates. </LI> <LI> The volcanism may be produced by Hainan plume, entraining the Paleo-Pacific slab. </LI> </UL> </P>

우리나라 상부암석권 맨틀

최성희(Sung Hi Choi) 한국암석학회 2012 암석학회지 Vol.21 No.2

우리나라 백령도, 제주도, 보은, 아산, 평택, 간성일대에는 맨틀 페리도타이트들이 알칼리 현무암류에 포획되어 온 형태로 분포하고 있다. K-Ar 전암연대측정 자료에 의하면 이들 화산암류는 약 0.1-18.9Ma에 분출하였다. 페리도타이트의 광물조합은 감람석-사방휘석-단사휘석-첨정석이며, 함수광물이나 석류석이 보고된 바는 없다. 암석의 종류는 레어조라이트에서 하즈버가이트에 걸쳐있다. 감람석은 Fo<SUB>88.4-92.0</SUB>이고, 단사휘석은 투휘석, 사방휘석은 엔스터타이트가 풍부하며, 첨정석은 Cr이 풍부하다(Cr# = 7.8-53.6). 주성분 원소함량에 의하면 이들 페리도타이트는 부화된 중앙해령현무암 근원 맨틀로부터 대략 26%까지의 부분용융을 겪은 후에 남은 잔류물로 추정된다. 그러나 맨틀에서의 이차적인 변성교대작용의 흔적이 미량원소의 함량에 기록되어 있는 경우도 있다. 양휘석 지온계에 의하면 우리나라 페리도타이트 포획암의 평형온도는 대략 850-1050℃ 범위이다. Sr-Nd 동위원소비는 결핍된 중앙해령현무암의 근원맨틀 성분(DMM)에서부터 지구전암(BSE) 성분 값까지 넓은 범위에 걸쳐있어, 오랜 시간의 성장과 진화의 역사를 대변하고 있다. Sr-Nd-Pb 동위원소비에 의하면 우리나라 암석권 맨틀은 대개 맨틀 단성분 중 DMM과 EM2의 혼합으로 설명되며, 상부 대륙지각과 연계될 수 있는 남북 내지는 동서방향으로의 지역성을 기록하고 있지는 않다. 이는 동아시아의 신생대후기 판내부기원 현무암류들이 기록하고 있는 남북간의 이분성과는 매우 대조적인 것으로, 동아시아 연약권 맨틀의 지역성을 시사하고 있다. Os 모델연대에 기초하면 우리나라 암석권 맨틀이 연약권으로부터 분리된 시기는 약 1.8-1.9 Ga로 추정된다. Peridotite xenoliths hosted by alkali basalts from South Korea occur in Baengnyeong Island, Jeju Island, Boeun, Asan, Pyeongtaek and Ganseong areas. K-Ar whole-rock ages of the basaltic rocks range from 0.1 to 18.9 Ma. The peridotites are dominantly lherzolites and magnesian harzburgites, and the constituent minerals are Fo-rich olivine (Fo<SUB>88.4-92.0</SUB>), En-rich orthopyroxene, Di-rich clinopyroxene, and Crrich spinel (Cr# = 7.8-53.6). Hydrous minerals, such as pargasite and phlogopite, or garnet have not been reported yet. The Korean peridotites are residues after variable degree of partial melting (up to 26%) and melt extraction from fertile MORB mantle. However, some samples (usually refractory harzburgites) exhibit metasomatic enrichment of the highly incompatible elements, such as LREE. Equilibration temperatures estimated using two-pyroxene geothermometry range from ca. 850 to 1050℃. Sr and Nd isotopic compositions in clinopyroxene separates from the Korean peridotites show trends between depleted MORB-like mantle (DMM) and bulk silicate earth (BSE), which can be explained by secondary metasomatic overprinting of a precursor time-integrated depleted mantle. The Korean peridotite clinopyroxenes define mixing trends between DMM and EM2 end members on Sr-Pb and Nd-Pb isotopic correlation diagrams, without any corresponding changes in the basement. This is contrary to what weobserve in late Cenozoic intraplate volcanism in East Asia which shows two distinct mantle sources such as a DMM-EM1 array for NE China including Baengnyeong Island and a DMM-EM2 array for Southeast Asia including Jeju Island. This observation suggests the existence of large-scale two distinct mantle domains in the shallow asthenosphere beneath East Asia. The Re-Os model ages on Korean peridotites indicate that they have been isolated from convecting mantle between ca. 1.8 and 1.9 Ga.

Petrogenesis of Late Cenozoic basaltic rocks from southern Vietnam

An, A.R.,Choi, S.H.,Yu, Y.,Lee, D.C. Universitetsforlaget ; Elsevier Science Ltd 2017 Lithos Vol.272 No.-

<P>Major and trace element concentrations, and Sr-Nd-Hf-Pb isotopic compositions of Late Cenozoic (4.1 to 13.8 Ma) basaltic rocks from southern Vietnam have been determined to understand the nature of their mantle source. The volcanic rocks are composed of tholeiite basalt, alkaline basanite, trachybasalt, basaltic trachyandesite, and trachyandesite. The alkaline rocks show light rare earth element (LREE) enrichment, with (La/Yb)(N) = 10.3-29.8. The tholeiite basalts are distinguished by much lower values (8.8-9.5) of (La/Yb)(N). On a primitive mantle-normalized trace element distribution diagram, they show oceanic island basalt (OIB)-like large-ion iithophile element enrichment without high field strength element depletion. However, some samples exhibit positive anomalies in K and Pb and negative anomalies in Sm, suggesting K-rich residual amphibole in the source. The samples contain Sr (Sr-87/Sr-86 = 0.703794-0.704672), Nd (epsilon(Nd) = + 1.7-5.7), Hf (epsilon(Hf) = + 4.0-10.9), and Pb ((206)pb/(204)pb, = 18.23-18.75;Pb-207/Pb-204 = 15.53-15.59; (208)pb/Pb-204 = 38.32-38.88) isotopes, plotting among OIBs, with depleted mid-ocean ridge basalt mantle-enriched mantle type 2 (DMM-EM2) characteristics. There are no discernible isotopic differences between tholeiite and the alkaline series, reflecting the same source. The Nd and Hf isotopic compositions are coupled, and plot along the mantle-crust array, ruling out the possibility of lithospheric mantle in the source. Plots of NiO against the Fo numbers of olivines from the basaltic rocks are within the range of Hainan and Hawaiian basalt olivines, implying that hybrid pyroxenite is present in the source. Also note that the estimated primary melt compositions fall within the experimental field defined by partial melting of silica-poor eclogite and peridotite. The effective melting pressure (P-f) and melting temperature (T) of the primary melts are P-f = 29.6-32.8 kbar and T = 1470-1480 degrees C. We suggest that Vietnamese basaltic rocks may be produced by the Hainan plume from recycled eclogitic oceanic crust. (C) 2016 Elsevier B.V. All rights reserved.</P>