Petrogenesis and tectonic setting of Early Cretaceous mafic dykes in the North Qinling Orogenic Belt, central China: constraints on the lithospheric lower crust delamination

Late Mesozoic mafic dykes, which are widely developed in the North Qinling Orogenic Belt (NQOB), include abundant geodynamic information. This paper describes the mafic dykes that intrude the Late Jurassic granite in the Dayu and Kuyu areas, and reports important petrological constraints for the late Mesozoic tectonic transition from compression to extension in the NQOB. Three zircon U–Pb results show that the minimum ages of the mafic dykes are 139.8 ± 1.4 Ma, 137.4 ± 1.7 Ma and 133.4 ± 0.9 Ma, indicating that the emplacement age of the Dayu and Kuyu mafic dykes is 140–133 Ma. Petrogeochemical analyses suggest that the mafic dykes belong to the high-K calc-alkaline shoshonite series with low SiO₂ (46.93–56.73 wt%), MgO (1.88–9.10 wt%) and TiO₂ (1.17–1.82 wt%), and high Al₂O₃ (13.98–17.46 wt%), TFe₂O₃ (7.81–10.92 wt%) and K₂O (1.28–4.78 wt%). The mafic dykes are enriched in large ion lithophile elements (<i>e.g.</i> Rb, Ba, K, La, Sr) and depleted in high-field-strength elements (<i>e.g.</i> Nb, Ta, Zr, Ti). These samples have the right-sloping chondrite-normalised rare earth element patterns, which suggest light rare earth element enrichment and heavy rare earth elements depletion with no obvious Eu anomalies (δEu = 0.94–1.11). The <i>I</i>_Sr, <i>ε</i>_Nd(t), <i>ε</i>_Hf (t) and <i>T</i>_DM2(crust) values are 0.7056–0.7060, −10.60 to −5.98, −14.1 to −2.8, and 1382.4 ± 25.1 to 2081.9 ± 47.6 Ma, respectively. Both elemental and isotopic geochemistry show that the formation of Dayu and Kuyu mafic dykes is due to the partial decompression melting of previously enriched lithospheric mantle during a delamination process. The mafic dykes have undergone fractionation crystallisation of Mg–Fe phase minerals during magma ascent, accompanied by some crustal contamination. Combined with the regional tectonic setting, we suggested that the NQOB experienced intra-continental extension during the Early Cretaceous.KEY POINTSEarly Cretaceous (140–133 Ma) mafic dykes have been discovered in the middle part of the North Qinling Orogenic Belt.The remote effect of the Paleo-Pacific Plate subduction has reached the middle of the North Qinling Orogenic Belt.The North Qinling Orogenic Belt entered the extensional stage in the Early Cretaceous (140–133 Ma). Early Cretaceous (140–133 Ma) mafic dykes have been discovered in the middle part of the North Qinling Orogenic Belt. The remote effect of the Paleo-Pacific Plate subduction has reached the middle of the North Qinling Orogenic Belt. The North Qinling Orogenic Belt entered the extensional stage in the Early Cretaceous (140–133 Ma).

北秦岭造山带（North Qinling Orogenic Belt, NQOB）广泛发育晚中生代镁铁质岩脉（mafic dykes），此类岩脉蕴含丰富的地球动力学信息。本文针对大峪（Dayu）与库峪（Kuyu）地区侵入晚侏罗世花岗岩的镁铁质岩脉展开研究，为北秦岭造山带晚中生代挤压向伸展的构造转换提供了关键岩石学约束。3组锆石U-Pb（zircon U–Pb）测年结果显示，该镁铁质岩脉的最小加权年龄分别为139.8±1.4 Ma、137.4±1.7 Ma及133.4±0.9 Ma，表明大峪和库峪镁铁质岩脉的侵位时代为140~133 Ma。岩石地球化学分析表明，研究区镁铁质岩脉属于高钾钙碱性-钾玄岩系列，具有低SiO₂（46.93~56.73 wt%）、MgO（1.88~9.10 wt%）与TiO₂（1.17~1.82 wt%），高Al₂O₃（13.98~17.46 wt%）、全铁（TFe₂O₃，7.81~10.92 wt%）及K₂O（1.28~4.78 wt%）的地球化学特征。岩脉富集大离子亲石元素（large ion lithophile elements, LILE，例如Rb、Ba、K、La、Sr），亏损高场强元素（high-field-strength elements, HFSE，例如Nb、Ta、Zr、Ti）。所有样品均呈现右倾的球粒陨石标准化稀土元素配分模式，表现为轻稀土富集、重稀土亏损，且无明显铕异常（δEu=0.94~1.11）。样品的初始锶同位素比值（I_Sr）为0.7056~0.7060，初始钕同位素值（ε_Nd(t)）为-10.60~-5.98，初始铪同位素值（ε_Hf(t)）为-14.1~-2.8，地壳二阶段模式年龄（T_DM2(crust)）为1382.4±25.1~2081.9±47.6 Ma。元素与同位素地球化学数据均显示，大峪和库峪镁铁质岩脉形成于拆沉过程中先前富集的岩石圈地幔（lithospheric mantle）的部分减压熔融（partial decompression melting）。岩脉在岩浆上升（magma ascent）过程中经历了镁铁相矿物的分离结晶作用（fractionation crystallisation），并伴随一定程度的地壳混染（crustal contamination）。结合区域构造背景，研究认为北秦岭造山带在早白垩世处于陆内伸展环境。【研究要点】1. 在北秦岭造山带中部发现了形成于早白垩世（140~133 Ma）的镁铁质岩脉；2. 古太平洋板块（Paleo-Pacific Plate）俯冲的远程效应已传递至北秦岭造山带中部；3. 北秦岭造山带在早白垩世（140~133 Ma）进入伸展构造阶段。早白垩世（140~133 Ma）镁铁质岩脉已在北秦岭造山带中部被发现。古太平洋板块俯冲的远程效应已波及北秦岭造山带中部。北秦岭造山带在早白垩世（140~133 Ma）进入伸展构造阶段。