Petrogenesis of Mesozoic High-Mg Diorites in Western Shandong: Evidence from Chronology and Petro-geochemistry

The high-Mg diorites, widely occurring in western Shandong, have important implications in the study of the relationship between the Mesozoic magmatism and the nature of the lithosphere and its thinning period in the eastern North China craton （NCC）. The Tietonggou and Jinling intrusions are typically representatives. LA-ICP-MS zircon U-Pb dating, major and trace element and Sr-Nd isotopic compositions of the Mesozoic intrusive rocks from western Shandong, eastern China, were analyzed. The weighted mean 106^pb/ 238^U ages from LA-ICP-MS zircon U-Pb dating results for early norite-diorite, late pyroxene-diorite from the Tietonggou intrusion and biotite-diorite from the Jinling intrusion are （131.4±4.9） Ma（n=15）, （134. 5±2.3） Ma（n=13）, and （132.8±4. 2） Ma（n=12）, respectively, implying that they were formed in the Early Cretaceous. The weighted mean 207^pb/ 204^Pb ages for round zircons from late pyroxene-diorite in the Tietonggou intrusion is （ 2 513 ± 54） Ma（ n= 8）, suggesting that the basement of the North China craton should exist in the research area. The high-Mg diorites are characterized by enrichment in MgO, Na20, and light rare earth elements （LREE）, and they are poor in heavy rare earth elements （HREE） and high field strength elements （HFSE）, being similar to adakite. The existence of the mantle peridotite xenoliths with a high-Mg feature for these intrusive rocks implies that their primary magma should be derived from the upper mantle. However, Sr-Nd isotopic composi- tions （Is： 0.704 75--0.707 15;ENd （130 Ma） values： --3.95 to -- 13.30）, depletion in HFSE, and the occurrence of Archean inherited zircons suggest that crustal materials could be involved in the derivation of the primary magma. The compositional difference between the diorites from the Tietonggou and Jinling intrusions could be attributed to those of magma sources and degrees of partial melting. The Early Cretaceous high-Mg diorites are considered to have been formed by the mixed melting of the delaminated lithosphere （lithospheric mantle ＋ crust） and asthenosphere, based on their geochemistry, the mantlederived xenoliths, and the Early Mesozoic lithospheric evolutionary history of the eastern North China craton.

Rare Earths, Trace Element Characteristics of High-Mg Volcanic Rocks of Yixian Formation in Sihetun West Liaoning Province and Its Apocalypse

The high-Mg volcanic rocks of the Yixian Formation in the Sihetun area have the obvious characteristics of mantie-derived lava in rare earth, trace element characteristics with high Mg # （62 - 70） and high content of compatible elements. In the meantime, the volcanic rocks also have the obvious characteristics of Crust-source material in rare earth, trace element characteristics with high ∑ REE （ 158.78 × 10^-6 - 359.66 × 10^-6 ）, high （La/Yb） N （ 14.61 - 29.60）, high La/Nb（2.37 - 7.52） and high Ba/Nb （67.58- 205.96）, obvious positive anomaly of Pb and negative anomaly of Nb, Ta in trace element spider-gram. In Sr-Nd-Pb isotope the （^87Sr/^86Sr）i ratio is higher than 0.706 and the εNd（ t ） ratio is from - 3.4 to - 13, both reflect enriched Mantle characteristics. The characteristics above of the volcanic rocks combined with the content of Sr, Ba, Y, Yb and the ratio of Sr/Y show that the volcanic rocks have the property of the Sanukite rocks in Setouchi Japan beside subduction zone, and illuminate that the Sanukite rocks can be formed not only in island-arc near subduction zone but also in intro-plate. The analysis indicates that the high-Mg volcanic rocks in the Sihetun area result from the collective function of mantle-derived lava and crust-source materials. The result illuminates that the West Liaoning region is very special in tectonic geochemical background in Cretaceous in East China, and is an ideal region for us to further study the characteristics of magmatic activity as well as the process of Crust-Mantle interaction in Eastern China.

Zircon U-Pb age,geochemical and Nd isotopic data of Middle Jurassic high-Mg dioritic dike in Liaodong Peninsula,NE China

Zircon LA-ICP-MS U-Pb age, geochemical and Nd isotopic data are presented for a newly recognized high-Mg dioritic dike from Haicheng, Liaodong Peninsula, NE China, to constrain its petrogenesis. The zircons from the high-Mg diorite exhibit striped absorption and oscillatory growth zoning in the cathodoluminescence （CL） images, and have high Th/U ratios （0. 05-0.9）, indicating a magma origin. Zircon LA-ICP-MS U-Pb dating indicates that 206pb/238U ages of 12 spots of zircons are between 167 Ma and 178 Ma, yielding a weighted mean 206pb/238U age of 172 ＋ 2 Ma （MSWD =4. 1 ）, which represents the forming age of the high-Mg dioritic dike, i. e. Middle Jurassic. Geochemically, the samples have SiO2 =55.4-60. 6 wt. % , Na20 =2. 2-2.76 wt. % , K20 = 1.32-2. 02 wt. % and （Na2O ＋ K2O） =3.82--4. 47 wt. %, belonging to sub-alkaline series and displaying a calc-alkaline evolutionary trend. They are characterized by high MgO （4. 75-6.85 wt. % ）, Mg# （55-61）, Cr（130-262 ppm）, Ni（63-130 ppm）, Sr（568-857 ppm）, and Ba（484-1 130 ppm） contents, with geochemical features analogous to those of high-Mg adakites. They show variable end （t） values （ - 1.3 to - 3.9） , with a weighted value of - 2. 7, which plot intermediately between the field of the ancient continental crust and the depleted mantle source, indicating that both the lower crust and mantle source are necessary for the generation of the parent magma of the Haicheng high-Mg diorites. The Haicheng high-Mg dioritic dike in the Liaodong Peninsula and the Jurassic magmatism in the eastern North China Craton formed under a continental crustal thickening setting that may be related to subduction of the Paleo-Pacific oceanic plate.

Discovery and Geological Significance of Early Carboniferous High-Mg Diorite in the Balikun Area, Eastern Tianshan

The granodiorites,monzogranites and diorites are widely developed in the Balikun area of Eastern Tianshan Orogen.Of which,LA–ICP–MS zircon U-Pb isotopic dating from the diorites revealed that they were emplaced at 327~333 Ma,representing an important period of magmatism in the Early Carboniferous.Geochemically,they are characterized by moderate SiO2(51.33–62.48 wt%),high but variable MgO(2.04–11.16 wt%,average 5.35),higher Mg#(40–73)and TiO2(0.67–1.29 wt%),Na2O/K2O(1.39–2.95)as well as variable Cr(2.49–675 ppm)and Ni(1.31–174 ppm),showing a geochemistry similar to those of high-Mg diorites or sanukitoids.In addition,they are enriched in the LILE,poor HFSE with an evident negative Nb anomalies and a REE pattern of moderate fractionator between LREE and HREE without or weak negative Eu anomalies.TheirεHf(t)are positive(+3.63–+15.65),suggesting a source from the depleted mantle.In addition,they have high TiO2 and Pb,and large quantity of amphibolite and biotite,indicating that they were most likely derived from the partial melting of depleted mantle metasomatized by the slabderived melt under a hydrous condition.Consequently,combined with the contemporary volcanics and granitoids formed in the island arc settings,we proposed that the subduction was continued till Early Carboniferous in the Bogda-Harlik tectonic belt.After that,wide occurrence of the post-collisional A-type granites and mafic-ultramafic intrusions indicate this tectonic belt entered the post-collisional environment from Late Carboniferous to Permian.

Geochemistry and Geochronology of the High-Mg Gabbro Dykes from Quanji Massif: Implications for the Amalgamation of Tarim Block in NW China and Assembly of the Rodinia Supercontinent

The Tarim Craton is one of the major continental blocks in China.Mafic dykes,ultra-mafic–mafic–carbonatite complex and ultra-mafic–mafic pluton of 0.82–0.75 Ga widely occur in Kuluktage,Aksu and Tieklik area along

Petrogenesis and Tectonic Significance of An Early Paleoproterozoic High-Mg Boninite-Norite-Diorite Suite of Rocks from the Bastar Craton, Central India

Mafic/ultramafic rocks derived from high siliceous(high-Si)high magnesium(high-Mg)mantle melts are an important feature observed during early Precambrian.They provide much important geological and petrological

鲁西中生代高Mg闪长岩的成因:年代学与岩石地球化学证据

鲁西高Mg闪长岩分布较为广泛,莱芜铁铜沟岩体和淄博金岭岩体是其典型代表．铁铜沟岩体早期苏长辉长岩和晚期辉石闪长岩、金岭黑云母闪长岩的LA-ICPMS U-Pb谐和加权平均年龄分别为(131．4±4．9)Ma(n=15)、(134．5±2．3) Ma(n=13)和(132．8±4．2)Ma(n=12),它们代表了岩体的侵位结晶年龄——早白垩世．铁铜沟岩体晚期辉石闪长岩中浑圆状锆石的LA-ICPMS U-Pb谐和年龄为(2 513±54)Ma(n=8),代表了华北克拉通基底的存在．该类岩石以高Mg# (>60)、富钠、富含轻稀土元素、贫重稀土元素以及高场强元素(Nb、Ta、Zr、Hf)的明显亏损为特征,显示埃达克岩(adakite) 的特征．该类岩石中地幔橄榄岩包体的广泛存在以及高Mg的特征表明原始岩浆的地幔成因,而Sr-Nd同位素组成(Isr为 0．704 75-0．707 72和εNd(t)值为-3．95--13．30)和高场强元素(Nb、Ta、Zr、Hf)的亏损以及太古代继承锆石的存在均暗示岩浆源区应有陆壳物质的涉入．铁铜沟岩体和金岭岩体闪长岩成分的差异应归因于岩浆源区性质的差异和部分熔融程度的不同．基于该类岩石的地球化学和所含包体,同时结合华北克拉通东部中生代早期岩石圈演化历史,可以认为鲁西中生代早白垩世高Mg闪长岩的形成应为拆沉的岩石圈(地幔+下地壳)与软流圈混熔的产物．

Mantle plume activity and melting conditions： Evidence from olivines in picritic-komatiitic rocks from the Emeishan Large Igneous Province, southwestern China

Olivine phenocryst compositions and whole-rock chemical compositions are used to identify primitive picrite basalts from the early part of flood basalt successions in the western part of the Emeishan large igneous province （ELIP）, southwestern China. The Fo contents of olivine phenocrysts of komatiites with MgO-21.79 wt% range from 85.2% to 91.4%. The composition of unerupted parental melts is calculated from the compositions of the most Fo-rich olivine phenocrysts. These melts had -22 wt% MgO, and originated in mantle with a potential temperature of -1600℃ and at about 4.2 GPa pressure, supporting the conclusion that the generation of ELIP can be attributed to melting of a plume head beginning at -138 km depth.

Eocene Crustal Thickening and Uplifting of the Tibetan Plateau:Evidence from Adakitic Volcanic Rocks

Changes in oceanic O-Sr-C isotopic compositions, global cooling and Asian continental aridification beginning in the Middle-Late Eocene(47-34 Ma) are considered to have been caused by the uplift of the Tibetan plateau.The specific timing and uplift mechanism,however,have long been subjects of debate.We investigated the Duogecuoren lavas of the central-western Qiangtang block,which form the largest outcrops among Cenozoic lavas in north-

Geochemistry of high-Mg andesites from the early Cretaceous Yixian Formation, western Liaoning: Implications for lower crustal delamination and Sr/Y variations

85 volcanic rocks of the Yixian Formation from the Sihetun type section werecollected and analyzed for geochemical and isotopic compositions. Major element compositionsindicate that the Sihetun volcanic rocks are high magnesium andesites with some basalts occurring atthe bottom of section. The Sihetun high magnesium andesites (SiO_2 = 52.82-59.31 wt%, Al_2O_3=14.15-16.35wt%) show many characteristics of adakites such as depletion in heavy rare-earth elements(HREE; Yb = 1.03-1.88 μg/g) and Y(12-20 μg/g) and high Sr (620-1323 μg/g) and Sr/Y(32-88), withhigh La_N/Yb_N ratio (10-25). They share similar major and trace element characteristics to volcanicrocks from the Xinglonggou Formation except their lower Nd isotope ratios (^(143)Nd/^(144)Nd(130Ma) = 0.5118-0.5119, ε_(Nd) (130Ma) = -11.6-13.8, ^(87)Sr/^(86)Sr (130 Ma) = 0.7058-0.7064.They were interpreted in a way that eclogite that formed at the base of thickened Archean lowercrust of the North China craton foundered into the convecting mantle and subsequently melted andinteracted with peridotite. However, compared to the Xinglonggou volcanic rocks, the source of theSihetun magma contained more ancient continental crustal material in order to explain its evolved Ndisotopes. The age of the Sihetun Formation was 120 to 130 Ma, and this indicates that delaminationlasted to the early Cretaceous period. The Sr contents and Sr/Y ratios of the Sihetun high-Mgandesites show significant negative correlations with SiO_2 for samples with SiO_2 】 56%. Thesesuggest that the Sr and Sr/Y values were reduced due to fractional crystallization of plagioclase.Accordingly, the effect of crystallization on volcanic Sr and Sr/Y ratio has to be taken intoaccount.

Reaction experiments between tonalitic melt and mantle olivine and their implications for genesis of high-Mg andesites within cratons

High-Mg （Mg#〉45） andesites （HMA） within cratons attract great attention from geologists. Their origin remains strongly debated. In order to examine and provide direct evidence for previous assumptions about HMA＇s genesis inferred from petrolog- ical and geochemical investigations, we performed reaction experiments between tonalitic melt and mantle olivine on a six-anvil apparatus at high-temperature of 1250-1400℃ and high-pressure of 2.0-5.0 GPa. Our experiments in this work simulated the interaction between the tonalitic melt derived from partial melting of eclogitized lower crust foundering into the Earth＇s mantle and mantle peridotite. The experimental results show that the reacted melts have very similar variations in chemical compositions to the HMAs within the North China Craton. Therefore, this interaction is probably an important pro- cess to generate the HMAs within crations.

Petrogenesis of Shangyu gabbro-diorites in western Shandong:Geochronological and geochemical evidence

Chronology and geochemistry of the Shangyu gabbro-diorite in western Shandong were studied to understand their petrogenesis and the nature of the Mesozoic lithospheric mantle. The Shangyu intru-sion is mainly composed of a suite of gabbro-diorite. Zircons from the intrusion display eu-hedral-subhedral in shape and have high Th/U ratios (1.23―2.87), implying their magmatic origin. LA-ICP-MS zircon U-Pb dating results for two samples indicate that they were formed in the Early Cre-taceous, yielding weighted mean 206Pb/238U ages of 129±1Ma and 134±2Ma, respectively. Except for early cumulate such as sample QT-19, their SiO2 and MgO contents range from 50.12% to 56.37% and from 3.52% to 6.37%, respectively. Moreover, the gabbro-diorites are characterized by high Mg# (0.54―0.63), enrichment in Na (Na2O/K2O ratios more than 1), Cr (73×10-6―217×10-6) and Ni (34×10-6―241×10-6), and intensive enrichments in light rare earth elements (LREEs) and large ion lithophile elements (LILEs) and depletion in high field strength elements (HFSEs). Their initial 87Sr/86Sr ratios and ε Nd(t) values range from 0.70962 to 0.71081 and from-16.60 to-13.04, respectively. Taken together with the Early Creta-ceous high-Mg diorites and the mantle xenoliths from the Tietonggou and Jinling as well as basalts from the Fangcheng and Feixian, it is suggested that the primary magma for the Shangyu gab-bro-diorites should be derived from the enriched lithospheric mantle intensively modified by conti-nental crust. The Sr-Nd-Pb isotopic compositions for the Early Cretaceous high-Mg diorites in western Shandong display a trend of spatial variations, i.e., initial 87Sr/86Sr, 207Pb/204Pb and 208Pb/204Pb ratios de-creasing and ε Nd(t) values increasing from southeast to northwest in western Shandong, which is con-sistent with the tectonic model that the Yangtze Craton subducted beneath the North China Craton oriented in north-west direction in the Early Mesozoic.