| The Luotuobozi pluton, located in the southeast of Haduohe area Great Xing’anRange, with its geotectonical position in the east of Xing’an-Mongolian Orogenic belt,with Xing’an and Songnen massif Neopaleozoic orogenic belt on its east. The plutonwas outputted as a batholith, and have an intrusive contact relationship with thesurrounding rock. The interior facies of the pluton is monzogranite granite. Thetransition facies is granodiorite and the marginal facies is syenogranite, each facies isclear and different from others. The structure of rock in this pluton is changed fromcoarse-grained to fine-grained. The relationship between each facies is gradualchanged.Such characteristics of zircons from sample imply their magmatic origin, aseuhedral shape, typical oscillatory zoning and high Th/U ratios. LA-ICP-MS zirconU-Pb dating indicates that the ages of zircons from Luotuobozi pluton aresyenogranite:127±1Ma, granodiorite:126±1Ma, dioritic enclaves:131±1Ma,monzonite:130±1Ma, concentrated in Early Cretaceous, not as the predecessorthought is concentrated in Late Permian.The granitic rocks is weak peraluminous belonging to high potassiumcalc-alkaline series, characterized by high Si (SiO2=68.89%~76.89%), high Alkali(Na2O+K2O=8.25%~9.63%), high aluminum (Al2O3=12.55%~15.48%) and lowtitanium (TiO2=0.08%~0.40%), with a higher alkalinity ration between2.81to5.70,A/CNK value0.94~1.04.The rock experiences a highly fractionated and evolvingstage reflected from DI (83.3~97.5).The rock are also rich in Th, U, Hf, Zr, La, Rband depleted in Ti, Ta, Nb., Zr+Nb+Ce+Y=44.01×10-6~294.50×10-6, they are lessthan that of the low-limit values of the A-type granites. Zircon saturation temperaturesof them range from720.68℃to806.08℃which are also less than that of thelow-limit values of the A-type granites.Combining petrography, rock geochemical characteristics, chronology, and regional geological feature, the Luotuobozi plutonbelongs to highly fractionated high-potassium calc-alkaline I-type granites, theprotolith of the granites was characterized by pelyte and greywacke which has beenpartial melting to a low degree under a unsaturated water condition, and has beenthrough a fractional crystallization with Sphene, Apatite and other mineral. Theformation of the granitic rocks is the magma derive from lower crust in partial melting,which mixed with mantle material then rising and emplacement with rocks indifferent depth and occurred as partial melting and magma contamination, whichcome into being different types of magma, then has been through a series of magmadifferentiation and evolutionary.The dioritic enclave exhibit the high Al(Al2O3=16.64%~17.29%), highNa(Na2O=3.56%~4.84%), high Ti(TiO2=0.78%~1.14%), low K(K2O=2.07%~2.53%),with Si(SiO2=55.14%~62.93%), alkalinity ration between1.58to2.08, A/CNKvalue0.79~0.92, belong to metaluminou. With the petrography and geochemistryindicate the cause of dioritic enclave’s formation is the residue of mafic magmamixed with felsic magma. The primary magma of this diorites were derived frompatial melting of mantle wedge which had been metasomatized by subduction fluidsand with less magma contamination in crust, then has been through a fractionalcrystallization with olivine and other mineral.Two types of rocks has a similar tectonic setting, the granitic rockswerecharacterized by subduction zone igneous rocks and post-collision granite, andlocated in an extension environment, the dioritic enclave meets the standards ofo-type adakitic rock, suggest the cause of formation is the subduction of paleo-Pacificplate and its melting, this also accord with the tectonic setting.In conclusion, the formation of this pluton is probably by the extensionalcontinental lithosphere thinning in northeast China after the subduction of the Pacificplate in Mesozoic. |
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