Petrogenesis of granitoids in the Wulan area: Magmatic activity and tectonic evolution in the North Qaidam, NW China

Numerous granitic intrusions crop out in the eastern segment of the North Qaidam block (NQ), NW China. To evaluate their ages, petrogenesis and genetic relationships to other granitoids in the NQ, we present geochemical and geochronologic data for six intrusive bodies and review regional data. Zircon U-Pb (SHRIMP) dating yielded ages of 413 ± 3 Ma for the Hadesengou granite; 254 ± 3 Ma for the Xugeigou granite; 251 ± 1 Ma for the Qiluoshan granite; 249 ± 1 and 248 ± 2 Ma for the Chahannuo hornblende diorite and granite, respectively; 240 ± 2 Ma for the Chahanhe granite; and 250 ± 1 and 244 ± 3 Ma for the Shailekegoulei granodiorite and granite, respectively. Consequently, the Wulan plutons can be divided into two petrologic groups: Early Devonian (D1) quartz monzonite and syenogranite, and Late Permian to Early Triassic (P3-T1) hornblende diorite, granodiorite, and granite. The D1 granitic intrusions have geochemical affinities with A-type granites (A2-type) characterized by low Ca, Sr, Ba and Nb, and high Fe, Ga, Y and Rb, consistent with derivation by partial melting of metapelitic source rocks containing a small amount of metagraywacke. The P3-T1 I-type granitic intrusions are geochemically typical of active continental margin rocks, consistent with derivation by partial melting of metabasalt and clay-poor metagraywacke. Combined with previous studies, we recognize five periods of granitic magmatism in the NQ: (1) 465–473 Ma; (2) 423–446 Ma; (3) 391–413 Ma; (4) 372–383 Ma; and (5) 240–271 Ma. Based on the temporal-spatial distribution of granitic intrusions in the NQ and the regional tectonic evolution, we interpret the first and second periods of granitic magmatism as related to normal plate subduction, and the third period to slab break-off and exhumation of the subducted plate. The fourth stage of granitic magmatism is attributed to large-scale lithospheric mantle delamination, involving the differential movement of orogenic blocks. The fifth period of granitic plutonism probably reflects northward subduction of the East Kunlun Paleotethys oceanic crust and southward subduction of Zongwulong oceanic crust beneath the Oulongbuluke continental block

Late Mesozoic high-K calc-alkaline magmatism in Southeast China: the Tongling example

<p>We report new zircon U–Pb ages, Hf isotopic and geochemical results for the Tongling granitic plutons of Southeast China. SHRIMP U–Pb ages for the Miaojia quartz monzodiorite porphyrite，the Tianebaodan and Tongguanshan quartz monzodiorites, the Xinqiaotou granodiorite porphyry, and the Shatanjiao and Nanhongchong granodiorite are 143 ± 2, 141 ± 1 and 142 ± 1, 147 ± 1, and 145 ± 1 and 139 ± 1 Ma, respectively. Combined with previous geochronological data, our results indicate that the porphyritic rocks are older than rocks of the same type lacking porphyritic texture. Geochemically, these high-K calc-alkaline intrusive rocks are characterized by arc-like trace element distribution patterns, with significant enrichment in LILE and LREE but depletion in HFSE. Lu–Hf isotopic compositions of zircons from the high-K calc-alkaline (HKCA) rocks have <i>ε</i>_Hf(<i>t</i>) values of magmatic 139–147 Ma zircons from −8.1 to −25.6, with two-stage model ages (<i>t</i>_DM2) of 1.71–2.67 Ga, whereas <i>ε</i>_Hf(<i>t</i>) values of inherited 582–844 Ma zircons range from 5.4 to −9.5, with <i>t</i>_DM2 of 1.39–2.22 Ma, younger than <i>t</i>_DM2 values of igneous zircon, indicating that newly formed mantle material was added to the continental crust of the Yangtze Block. Moreover, <i>ε</i>_Hf(<i>t</i>) values of inherited zircon cores older than 1000 Ma are from −7.8 to −26, similar to magmatic zircons, and the <i>t</i>_DM2 values are all greater than 3.0 Ga (3.16–3.75 Ga), reflecting partial melting of ancient sialic material. We conclude that the plutonic melts were derived from both the enriched mantle and the ancient crust. The HKCA Tongling intrusions coincide temporally with the J₃–K₁ magmatic event that was widespread in Southeast China. This igneous activity may have accompanied sinistral slip along the Tan-Lu fault due to oblique subduction of the Palaeo-Pacific plate.</p

Age, composition, and tectonic significance of Palaeozoic granites in the Altyn orogenic belt, China

<div><p>ABSTRACT</p><p>We examined the petrography, litho-geochemistry, and geochronology of granite plutons in the Altyn orogenic belt, northern margin of the Qinghai–Tibet Plateau, to investigate the geodynamic history of the belt. The granites are peraluminous, with variable chondrite-normalized rare earth element patterns but generally similar trace element compositions; all are depleted in Ba, Nb, Sr, P, and Ti, and enriched in Rb, Th, Ta, Zr, and Hf. Their U–Pb zircon ages record multiple magmatic episodes between 500 and 404 Ma (north Altyn block (NAB)), 522 and 432 Ma (central Altyn block (CAB)), and 483 and 226 Ma (south Altyn block (SAB)). We propose a geodynamic model for the Altyn orogenic belt. Prior to 467 Ma, the north Altyn Ocean subducted southward below the NAB. The north Altyn region saw a change from subduction to collision between the Dunhuang block and the NAB during 467–450 Ma. The collision lasted until 430 Ma and was followed by post-collisional extension until 400 Ma. Farther south, the central Altyn Ocean subducted northwards beneath the CAB, resulting in 520–500 Ma granitic magmatism and leading to collision of the south Altyn micro-block (SAMB) with the CAB. Deep subduction of SAMB crust below the CAB resulted in granite intrusions during 490–455 Ma. The SAMB–CAB collision gave way to post-collisional extension, slab break-off, and delamination during 450–430 Ma. Bidirectional subduction of the south Altyn Ocean after ~485 Ma, dominated by southward subduction, continued until 456 Ma. Closure of the south Altyn Ocean led to continent–continent collision between the SAMB and the Qaidam block at 450–430 Ma, followed after ~410 Ma by repeated break-offs of slab segments until 337 Ma. Subsequent intrusions formed at 264–226 Ma in a setting of either post-collisional extension or sinistral strike-slip.</p></div