The Origin Of The Houshihushan Alkaline Ring Complex In The Yanshan Orogenic Belt And Its Tectonic Implications

Understanding the temporal and spatial evolution of Magma plumbing systems is a significant part of our search for a better understanding of how continents grow and evolve because the continental crust initially formed and was subsequently influenced by magmatic activity. Ring complexes are direct representations of shallow magma pathways beneath calderas that provide a rich source of information about the evolution of caldera/volcano root zones and roof regions of magma chambers within the upper crust. The Yanshan Orogenic Belt is an intracontinental orogenic belt along the northern margin of North China craton that was the site of very active volcanism during a short interval of Late Mesozoic times that gave rise to a series of calderas. Their collapse together with simultaneous or subsequent magmatic intrusion formed a large number of subvolcanic ring complexes containing a variety of intrusive structures, some not previously recognized. The present work focusses on the representative Houshihushan subvolcanic Ring Complex, making a detailed study of aspects of geology, petrography, geochronology and geochemistry, using details of internal structure to elucidate its origin and explore its evolution as subvolcanic magma system. The results provide constraints on the geodynamic setting of late Mesozoic magmatic activity on the northern margin of northern China. We have also conducted a comparative study of the Houshihushan Ring Complex (HRC) and its contemporary neighboring Changli Batholith to gain a better understanding of the temporal and spatial distribution of Early Cretaceous magmatism in the Yanshan Orogenic Belt and its dynamic mechanisms.Field geological surveys and petrographic studies indicate that the HRC is a typical volcanic-intrusive ring complex, consisting of an inner ring of the Zhangjiakou Volcanic Formation, an outer ring pluton of porphyritic quartz syenite, a central composite hypabyssal intrusion of nested stocks of drusy alkaline granites, and cone sheets of quartz syenite porphyry and granitic porphyry, with a previously undiscovered diabase dyke intruding the porphyritic quartz syenite pluton a little later than the alkaline granite emplacement. Porphyritic quartz syenite and alkaline granite show granophyric texture (graphic texture) with miarolitic structure, and contain phenocrysts with embayed borders. The matrixes of quartz syenite porphyry and granitic porphyry are cryptocrystalline or glassy, and these porphyry dykes have chilled contacts and internal flow structure. All the above characteristics indicate the rocks are of hypabyssal intrusive facies. Some quartz syenite porphyries have distinctive internal spherulitic structures with contrasting compositions between spherulites and matrix which may indicate that they have experienced magma mixing or liquation. LA-ICP-MS zircon U-Pb dating indicates that porphyritic quartz syenite, drusy alkaline granite, quartz syenite porphyry and granitic porphyry were intruded at119±3Ma,118±1Ma,121±2Ma and121±2Ma respectively. Zircon U-Pb analyses yielded mean206Pb/238U ages of119±2Ma for trachytic dacite, which is identical to other alkaline rocks in the HRC within experimental error. Volcanic rocks of the Houshihushan Ring Complex (HRC) have similar ages to those of the intusive rocks, confirming it as a volcanic-intrusive complex. The diabase dyke shows an emplacement age of120Ma, coeval with the complex. The nearby Changli Batholith mainly consists of early medium to fine-grained biotite-bearing K-feldspar granite, and later medium to coarse-grained hornblende/biotite-bearing K-feldspar granite. LA-ICP-MS zircon U-Pb dating indicates the hornblende/biotite-bearing K-feldspar granite was emplaced at119±2Ma.The rocks from the HRC can be divided into three groups according to their petrographic and geochemical features. Group1rocks consist of granite porphyry, quartz syenite porphyry, akali-feldspar granite, rhyolite and trachy dacite characterized by relatively high alkali, FeOtot/MgO, Ga/Al, Zr, Nb and REE (except for Eu), low Al2O3,CaO and MgO and obvious negative Ba, Sr, P, Eu and Ti anomalies, similar to A-type granites. The quartz syenite porphyry and rhyolite are similar to aluminous A-type granites that would have high initial magma temperatures (854～930℃). However, the granite porphyry and akali-feldspar granite are similar to peralkaline A-type granites. Enrichment of Th and U and variable Sr-Nd-Hf isotopic compositions indicate these rocks might have been generated by partial melting of upper crustal calc-alkaline granites and then mixed with contemporaneous magma from EMI-type lithospheric mantle and lower crust, accompanied by fractionation of plagioclase and biotite. Group2rocks are composed of diabase with SiO2=47.8%～49.2%, TFe2O3=9.74%～10.9%, Cr=177-196×10-6, Ni=64.7～81.2×10-6, Mg#=50.6～53.3, slight enrichment of LREEs, flat HREEs, depletion of HFSEs, Th and U, weak negative Eu anomalies (Eu/Eu*=0.9～1.0), variable initial87Sr/86Sr ratios (0.7058～0.7110) and moderately enrichedεNd(t)(-6.5～-6.4). The group2rocks might be derived from partial melting of EMI-type lithospheric mantle with subsequent lower crustal contamination and some degrees of fractionation of apatite and Fe-Ti oxides. Group3rocks consist of (porphyritic) quartz syenite and trachyte characterized by relatively variable SiO2(55.7%～65.6%), A12O3(15.8%～16.5%) and Na2O+K2O (7.44%～10.5%), high K2O (2.99%～5.42%), low Mg#(20.6～37.6), enrichment of LILEs, LREEs and Pb, depletion of HFSEs, Th and U and no significant Eu anomalies (Eu/Eu*=0.75～1.40). All these rocks have (87Sr/86Sr)i= 0.7057～0.7099, εNd(t)=-14.4～-13.0, εHf(t)=-19.5～-11.1, and Nd-Hf model ages of1.97～2.09Ga and1.88～2.41Ga, respectively. Chemical and isotopic compositions indicate the Group3rocks might be derived from mixing of two different melts which formed by partial melting of old intermediate-basic granulites/genisses and EMI-type lithospheric mantle, respectively. The initial magma underwent fractionation of clinopyroxene, apatite and titanite prior to the emplacement.The SiO2contents of the studied Changli K-feldspar granites range from72.85%to76.51%and show typical characteristics of A-type granites, e.g. high alkali contents and low A12O3and significant negative anomalies in Ba, Sr, P, Eu and Ti. All our samples are highly fractionated Ⅰ-type granites by comparison with world average A-type granites and highly fractionated Ⅰ-type granites in northeastern China and the Lachlan fold belt in southeastern Australia. These Changli highly fractionated I-type granites have different elemental geochemical and Sr-Nd-Hf isotopic features compared with the A-type granitic rocks from the HRC, indicating that they have a different petrogenesis. The biotite-bearing K-feldspar granites have high147Sm/144Nd ratios (-0.14) and display lower LREE contents such as Th, U and HFSEs (e.g. Nb and Ta). In addition, the K-feldspar granites show low εNd(t)=-15.5and abnormally low (87Sr/86Sr)i=0.6996, with TDM2=2.17Ga. All these geochemical features indicate that these granites probably originated from partial melting of ancient lower crustal intermediate-basic granulites and then underwent strong fractional crystallization of plagioclase, apatite and Fe-Ti oxides. In contrast, medium to coarse-grained hornblende/biotite-bearing K-feldspar granites show (87Sr/86Sr)i=0.7049～0.7065andεNd(t)=-14.8～-14.6, with TDM2=(2.10～2.12Ga), exhibit zircon εHf(t) values of-12.9～-9.4and Hf model ages of average crust (TDMC) ranging from1.77to1.99Ga. Their chemical and isotopic characteristics indicate that they could be derived from partial melting of ancient lower crust mixed with magma from EMI-type lithospheric mantle magma, with subsequent fractionation of plagioclase, apatite and Fe-Ti oxides.On the basis of field research, petrology and geochemistry of the HRC combined with the chemical evidence of petrogenesis, we suggest that the evolution of the HRC involved the following four-stage sequence:1) the majority of alkaline lavas and pyroclastics were erupted explosively;2) a caldera subsided by loss of magma from an underlying magma chamber which reduced magma pressure and facilitated collapse of the roof of the magma chamber along near-vertical ring faults. Magma was intruded passively up opening ring-faults to form the ring dyke of porphyritic quartz syenite during caldera collapse;3) the high-level magma chamer became overpressured and hot peralkline A-type granite magma was emplaced as the central stock of porphyritic alkali-feldspar granite. The overlying crust was fractured to generate cone fractures that provided space for the ascent of felsic melts giving rise to cone sheets of quartz syenite porphyry;4) the chamber refilled with magma and a cogenetic pluton was emplaced as a nested stock of drusy alkali-feldspar granite. Build-up of magma overpressure within the central source chamber imparted upward force to fracture the host rock and form new conical fractures that filled with magma to form cone sheets of granite porphyry.We have summarized the temporal and spatial distribution of magmatism in the Liaodong Peninsula, Jiaodong Peninsula as well as the Luxi Region and found that the I-type and A-type granites, alkaline rocks, and basic intrusions are coeval with Early Cretaceous formation of extensional rift basins and metamorphic complexes, bimodal volcano eruptions and gold mineralization. These represent an extensional tectonic regime related to the destruction of the North China Craton, which could have been induced by the subduction of the Pacific Plate. Pacific Plate subduction and then rollback took place in the Early Cretaceous, causing extension of the eastern North China Craton and extensive asthenosphere upwelling, indicating the peak has reached for the craton destruction.