A Geochemical Study Of Mesozoic Magmatic Rocks In The Sulu Orogen

The Dabie-Sulu orogenic belt formed by the Triassic continental collision between the South China Block and the North China Block, with voluminous occurrence of postcollisional magmatic rocks. These mgamtic rocks contain valuable information on chemical geodynamics of continental subduction. Therefore, studies concerning the timing, rock type, geochemical characteristics, magma source, and geodynamic setting of postcollisional magmatic rocks have important implications for understanding the recycling of subducted continental crust and the tectonic evolution of collisional orogen. While much attention has been paid to postcollisional magmatic rocks in the Dabie orogen with respect to the recycling of the deeply subducted continental crust, fewer studies and progresses have been made on this topic for postcollisional magmatic rocks in the Sulu orogen. The Sulu orogen is one of most important regions in east-central China that crop out Mesozoic magmatic rocks, with emplacement ages mainly at Late Triassic, Late Jurassic and Early Cretaceous. This study deals with the Mesozoic granitoids from both the eastern Sulu orogen (the Jiaodong terrane) and the Jiaobei terranes as well as mafic to felsic dykes in the western Sulu orogen (the Subei-Jiaonan terrane).This dissertation presents a combined study of petrography, whole-rock major, trace elements and Rb-Sr and Sm-Nd isotopes, whole-rock and mineral O isotopes as well as zircon trace elements, U-Pb and Lu-Hf isotopes for the Mesozoic magmatic rocks in the Sulu orogen. The results not only provide insight into the source nature and petrogenesis of Mesozoic magmatic rocks in the Sulu orogen, but also place important constraints on the chemical differentiation and crust-mantle interaction of orogenic lithosphere.Four plutons were investigated, which are the Linglong and Guojialing plutons from the Jiaobei terrane and the Kunyushan and Sanfoshan plutons from the Jiaodong terrane. SHRIMP and LA-ICPMS zircon U-Pb dating yields Late Jurassic ages of 141±3 to 157±2 Ma for the Linglong and Kunyushan plutons, but Early Cretaceous ages of 111±2 to 133±3 Ma for the Guojialing and Sanfoshan plutons. Most of the granitoids have similar REE and trace element patterns with LREE and LILE enrichement but HREE and HFSE depletion as well as positive Pb anomaly. Most of them have high Sr, low Yb and Y contents as well as high (La/Yb)N and Sr/Y ratios with negligible to positive Eu anomalies (Eu/Eu*=0.69 to 1.58), which are similar to common adakites. This indicates the presence of garnet or garnet and amphibole as residual phases during partial melting. On the other hand, they have relatively low MgO, Cr, Ni contents and thus low Mg#. In addition, they have high initial 87Sr/86Sr ratios of 0.7080 to 0.7139 and lowεNd(t) values of-22.3 to-10.9 as well as zirconεHf(t) values of-39.6 to-5.4. They mostly have high zirconδ18O values of 5.91 to 8.93‰.The results of whole-rock major and trace elements, radiogenic isotopes and mineral O isotopes indicate that these Mesozoic granitoids were derived from partial melting of ancient continental crust. All the granitoids have variably negative zirconεHf(t) values of-39.6 to-5.4, with Paleoproterozoic Hf model ages of 2097±63 to 2387±21 Ma for the Sanfoshan pluton, but mainly Paleoproterozoic to Neoarchean Hf model ages of 2277±39 to 2856±44 Ma for the other three plutons. In addition, some zircon cores from the Linglong and Kunyushan granitoids have middle Neoproterozoic U-Pb ages of 769±19 to 790±21 Ma. The middle Paleoproterozoic crustal growth and middle Neoproterozoic magmatism are diagnostic features that distinguish the South China Block from the North China Block. Therefore, the Mesozoic granitoids were mainly derived from partial melting of the subducted continental crust of the South China Block. However, there are some differences between the Jiaobei and Jiaodong plutons. Compared to the Jiaodong granitoids, the Jiaobei granitoids have very old zircon Hf model ages up to 3310±96 Ma, suggesting the possible involvement of very ancient continental crust of the North China Block. Thus, the continental collision between the two blocks would bring crustal materials from both sides into the subduction zone during the Triassic, yielding subduction-thickened crust as the magma source for the adakite-like granitoids.Postcollisional mafic to felsic dykes in the Jiaonan terrane consist of monzogranite, diorite, plagioclase-bearing hornblendite and gabbro. LA-ICPMS zircon U-Pb dating yields broadly consistent ages of 111±3 to 129±1 Ma for magma crystallization of these mafic to felsic rocks, with Neoproterozoic ages of 743±9 to 773±9 Ma for residual cores in some monzogranite samples. The mafic to felsic rocks are characterized by strong LILE and LREE enrichment and negative HFSE anomalies, high initial 87Sr/86Sr ratios of 0.7079 to 0.7100 and very lowεHf(t) values of-21.6 to-13.6. Zircon Lu-Hf isotope analyses also show highly negativeεHf(t) values of-36.7 to-10.6, corresponding to two-stage Hf model ages of 1.84 to 3.47 Ga. They have relatively limited range of zirconδ18O values of 4.57 to 5.98%o, some of them are slightly higher or lower than those of the normal mantle.The arc-like continental crustal signatures are evident for the mafic dykes, suggesting their derivations from enriched mantle sources, which may be generated by interaction between the overlying subcontinental lithospheric mantle (SCLM) and felsic melts derived from the subducted continental crust during the Triassic subduction. However, the difference in geochemical and isotopic features between gabbro and plagioclase-bearing hornblendite indicates that they were derived from different SCLM sources. The gabbros have relatively high MgO, Cr, Ni and low Al2O3 contents, indicating that they were derived from an orthopyroxene-rich SCLM source. The plagioclase-bearing hornblendites have relatively high Al2O3, K2O and low MgO, Cr, Ni contents, indicating that they were derived from a hornblende-rich SCLM source. The general similarity in geochemical and isotopic characteristics between the felsic-intermediate dykes (monzogranite and diorite) and widespread postcollisional Early Cretaceous granitoids in the Dabie-Sulu orogenic belt indicate their derivation from the similar source materials, i.e. the subducted continental crust of the South China Block. Some felsic-intermediate samples have high Mg# up to 66, suggesting that they were possibly formed by mixing between mafic and felsic magmas. Therefore, the mafic to felsic dykes were formed by partial melting of the collision orogenic lithosphere in the Early Cretaceous.In summary, the Mesozoic magmatic rocks in the Sulu orogen were derived from partial melting and recycling of the subducted continental crust of the South China Block. Mantle heat perturbation due to extension of the continental lithosphere triggered partial melting of orogenic crust and underlying SCLM, resulting in the postcollisional magmatic rocks with the chemical and isotopic compositions of arc-like continental crust.