Monazites from low-pressure pelitic granulite of the Jingshan Group in the Jiaobei terrane were dated by the electron microprobe method. Three stages of metamor- phic age at 1720±15 Ma, 1687±16 Ma and 1568±15 Ma were yielded by constructing age mapping for zoned mona- zites and PbO-ThO2* isochron diagram for unzoned mona- zites, respectively. The first age was interpreted as the age of an early amphibolite-facies metamorphism, the second age as the peak granulite-facies, and the last age as uplifting of the granulite unit. These ages are the first geochronologic data for the low-pressure pelitic granulites in the Jingshan Group, and thus are of important significance when discussing the tectono-metamorphic evolution of the Jiaobei terrane.
In this paper we report geochemical and Nd-Sr isotopic data for a late Archean gneissic granitic pluton (Hengling pluton), an early Paleoproterozoic complex (Xipan complex) and a late Paleoproterozoic granitic pluton (Yunzhongshan granites) from the Lüliang-Wutai terrain, North China, to trace the source of these late Archean-Paleoproterozoic granitoids and, particularly, to understand the nature and mechanism of continental growth at that time. The Hengling granitic gneisses (ca. 2.51 Ga) are characterized by high Na2O and LILEs, TTG-like REE patterns (highly depleted HREE and minor Eu anomalies) and moderately depleted Nd-Sr isotopic compositions (εNd(t) =1.2?2.7, ISr=0.7015?0.7019), and were considered as being products of arc magmatism that was developed upon the North China craton. The Xipan complex (ca. 2.2 Ga) contain gabbroic diorite and monzonite, mostly being Na2O-rich, highly fractionated REE patterns and isotopically enriched (εNd(t) =?1.5 to ?4.1, ISr=0.7038?0.706). The gabbroic diorites probably originated from melting of an enriched mantle source, but significantly contaminated by lower crustal material, and the monzonites probably represent a product of a mixture between the gabbroic dioritic magma and granitic melts of crustal origin. The Yunzhongshan post-collisional granitoids (ca. 1.8 Ga) are characterized by high-K affinity and highly-enriched and homogeneous Nd isotopic compositions (εNd(t)=?4.9 to ?5.7), although they split into two groups in terms of REE patterns: one group showing elevated HREE (and Sc, Y and Zr) with significant negative Eu anomalies and the other showing highly depleted HREE and, to a lesser extent, mid-REE with negligible Eu anomalies. These granites are genetically related to a process of extensional collapse of a thickened orogen. They formed through magma mixing between mantle- derived basaltic magmas and crust-derived granitic melts, followed by significant fractionation of ferromagnesian phases (like hornblende and Cpx) and feldspar and accessory zircons. Some
CHEN Bin, LIU Shuwen, WANG Rui, CHEN Zhichao and LIU Chaoqun Key Laboratory of Orogenic Belts and Crustal Evolution, MOE
Neoarchean metamorphic mafic rocks in the lower and the middle Wutai Complex mainly comprise metamorphic gabbros, amphibolites and chlorite schists. They can be subdivided into three groups according to chondrite normalized REE patterns. Rocks in Group #1 are characterized by nearly flat REE patterns (Lan/Ybn=0.86?1.3), the lowest total REEs (29?52 ppm), and weak negative to positive Eu anomalies (Eun/Eun*=0.84?1.02), nearly flat primitive mantle normalized patterns and strong negative Zr(Hf) anomalies. Their geochemical characteristics in REEs and trace elements are similar to those of ocean plateau tholeiite, which imply that this group of rocks can represent remnants of Archean oceanic crust derived from a mantle plume. Rocks in Group #2 are characterized by moderate total REEs (34?116 ppm), LREE-enriched (Lan/Ybn=1.76?4.34) chondrite normalized REE patterns with weak Eu anomalies (Eun/Eun*=0.76?1.16), and negative Nb, Ta, Zr(Hf), Ti anomalies in the primitive mantle normalized spider diagram. The REE and trace element characteristics indicate that they represent arc magmas originating from a sub-arc mantle wedge metasomatized by slab- derived fluids. Rocks in Group #3 are characterized by the highest total REEs (61?192 ppm), the strongest LREEs enrichment (Lan/Ybn=7.12?16) with slightly negative Eu anomalies (Eun/ Eun*=0.81?0.95) in the chondrite normalized diagram. In the primitive mantle normalized diagram, these rocks are characterized by large negative anomalies in Nb, Ta, Ti, negative to no Zr anomalies. They represent arc magmas originating from a sub-arc mantle wedge enriched in slab-derived melts. The three groups of rocks imply that the formation of the Neoarchean Wutai Complex is related to mantle plumes and island-arc interaction.
Lü YongjunLIU ShuwenGuochun ZHAOLI QiugenJian ZHANGLIU ChaohuiK. H. PARKY. S. SONG
The Paleoproterozoic L(u|¨)liang Metamorphic Complex(PLMC) is situated in the middle segment of the western margin of the Trans-North China Orogen(TNCO),North China Craton(NCC). As the most important lithological assemblages in the southern part of the PLMC,Guandishan granitoids consist of early gneissic tonalities,granodiorites and gneissic monzogranites,and younger gneissic to massive monzogranites.Petrochemical features reveal that the early gneissic tonalities and granodiorites belong to the medium-K calc-alkaline series;the early gneissic monzogranites are transitional from high-K calc-alkaline to the shoshonite series;the younger gneissic to massive monzogranites belong to the high-k calc-alkaline series,and all rocks are characterized by rightdeclined REE patterns and negative Nb,Ta,Sr,P,and Ti anomalies in the primitive mantle normalized spidergrams.SHRIMP zircon U-Pb isotopic dating reveals that the early gneissic tonalities and granodiorites formed at～2.17 Ga,the early gneissic monzogranites at～2.06 Ga,and the younger gneissic to massive monzogranites at～1.84 Ga.Sm-Nd isotopic data show that the early gneissic tonalities and granodiorites haveε_(Nd)(t) values of +0.48 to -3.19 with Nd-depleted mantle model ages (T_(DM)) of 2.76-2.47 Ga,and early gneissic monzogranites haveε_(Nd)(t) values of-0.53 to -2.51 with F_(DM)of 2.61-2.43 Ga,and the younger gneissic monzogranites haveε_(Nd)(t) values of -6.41 to -2.78 with a T_(DM)of 2.69-2.52 Ga.These geochemical and isotopic data indicate that the early gneissic tonalities, granodiorites,and monzogranites were derived from the partial melting of metamorphosed basaltic and pelitic rocks,respectively,in a continental arc setting.The younger gneissic to massive monzogranites were derived by partial melting of metamorphosed greywackes within the continental crust.Combined with previously regional data,we suggest that the Paleoproterozoic granitoid magmatism in the Guandishan granitoids of the PLMC may provide the best geological signature for the complete
LIU Shuwen LI Qiugen LIU Chaohui LU Yongjun ZHANG Fan
