Pre-Hercynian Crustal Anatexis In The Cathaysia Block

The Cathaysia Block, located in the southeast part of South China, expose extensive Precambrianmetamorphic basement rocks that were heavily reformed by later geological events. Thebasement metamorphic rocks experienced later strong structural deformation and migmatizationin SW Zhejiang and NW Fujiang Provinces in the interior of the Cathaysia Block, with extensiveexposed granitic complex in it. The granitic rocks of the metamorphic basement provide awindow into the study of pre-Hercynian crustal anatexis and tectonic evolution of the CathaysiaBlock. Based on field observation, systematic sampling and petrographic investigation, acombined study of rock geochemistry, zircon trace elements, U-Pb and Lu-Hf isotopes wascarried out on the granitic rocks in basement rocks of SW Zhejiang and NW Fujian Provinces,aiming to elucidate the timing and genesis of pre-Hercynian crustal anatexis in the CathaysiaBlock and its tectonic significance.1. The Danzhu and Sanzhishu granites, exposed in the Badu Group of SW ZhejiangProvince, are biotite K-feldspar granite and adamellite, respectively. They are high in Si, alkaliand K, and low in Al, Ca and Mg. The granites are metaluminous with high FeO^*/MgO and10000Ga/Al ratios and high (Zr+Nb+Ce+Y) content, implying that they are typical aluminousA-type granites rather than S-type granite as previously thought. LA-ICP MS U-Pb datinganalyses of magmatic zircons reveal that the formation ages of the Danzhu and Sanzhishugranites are 1844±10 Ma and 1860±13 Ma, respectively. The magmatic zircons had negativeε_Hf(t)values of -5.4--3.6 and -15.6 to -10.0 for each, suggesting the granites are anatetic products ofancient crust.2. The granitic rock in Dazhe area of SW Zhejiang Province, previously regarded asgranodiorite, is actually orthopyroxene-bearing granitic granulite, with metamorphic age of1851±11 Ma. The metamorphic zircons had negativeε_Hf(t) values of -13.4--9.5. Along with thegeographic observation of biotite dehydration melting reactions in the granulite, we suggest thatthe Dazhe granitic granulite was likely formed by anatexis of ancient crust during the granulite facies metamorphism. Combining A-type affinities of the Danzhu and Sanzhishu granites withthe retrogressive metamorphism evolution of the Dazhe granulite characterized by near-isobariccooling (IBC), the～1.85 Ga mafic underplating in the Cathaysia Block might be responsible forthe granulite facies metamorphism (probably accompanied by crustal extension) and the meltingof ancient crust that produced the granites. The～1.85 Ga magmatic-metamorphic events in theCathaysia Block may mark the transition from assembly to break-up of the Columbiasupercontinent.3. On the basis of detailed studies of geochemistry and zircon geochronology, there existed agenetic link among the basement rocks, migmatites and granites in NW Fujian Province. Theoriginal granitic melts were likely produced by dehydration-melting of the biotite frommetasedimentary rocks in basement under relatively low temperature (～800℃) andH₂O-undersaturated conditions, then the leucosomes of migmatites were probably formed by thefractional crystallization and crystal accumulation of the initial granitic melts mentioned above,and the remaining melts continued to evolve and ultimately crystallized as granites. Themetamorphic ages of the basement metamorphic rocks fall in the range of 455-443 Ma, and thecrystallization ages are～437 Ma for the leucosomes of migmatites and～440 Ma for the granites.The zircon Hf isotope data reveal that breakdown of pre-existing zircons and minerals other thanzircon in the source played a key role in new zircon formation by release of Hf into the melt.Meanwhile, the newly formed zircons had negativeε_Hf(t) values and high two-stage Hf modelages, indicating that the basement metamorphic rocks, migmatites and related granites in NWFujian were formed by the Caledonian crustal anatexis and reworking event in South China.4. The Caledonian metamorphism, anatexis and magmatism observed in the metamorphicbasement of NW Fujian Province was a protracted tectonothermal event which lasted for ca. 15Mya, and the migmatites and related granites might be a result from an intracontinental collisionbetween the Yangtze and Cathaysia blocks. Combined with previous research, the initiation ofCaledonian orogeny in South China began no later than 458 Ma, with climax of collision at～443Ma and collapse of the orogenic belt slightly later than 423 Ma.5. The ages of 1.4～1.3 Ga recorded by inherited zircons from the granitic rocks in NWFujian Province are identical to the formation ages (1.44～1.43 Ga) of the magmatic rocks onHainan Island at the southern end of the Cathaysia Block, and also synchronous with the1500-1350 Ma granite-rhyolite in southern Laurentia. In fact, the zircon age spectra ofNeoproterozoic sediments in NW Fujian Province is quite similar to that of river sands fromwestern Laurentia (North America), providing good evidence for a connection between Cathaysiaand Laurentia in Rodinia. Besides, whether there was a Grevillian orogenic belt in the southernCathaysia Block is open to discussion.