Abstract Field relationships, mineralogy and petrology, whole‐rock chemistry, and age of the Zhamashi mafic–ultramafic intrusion in the North Qilian Mountains, northwest China, have been studied in the present work. The Zhamashi intrusive body consists of ultramafic, gabbroic, and dioritic rocks in a crudely concentrically zoned structure. The ultramafic rocks are layered cumulates with rock types varying continuously from dunite through wehrlite and olivine clinopyroxenite to clinopyroxenite. The gabbroic and dioritic rocks are also layered or massive cumulates with rock types varying continuously from noritic gabbro through hornblende gabbro to diorite. The ultramafic and adjoining gabbroic rocks are discontinuous in lithology and discordant in structure across the interface. The interface is steep, sharp, and fractured. Contact metamorphic zones are well developed between the Zhamashi intrusive body and the country rock. The concentrically zoned structure of the intrusive body and the intrusion into the continental crust are the two main pieces of evidence for considering that the Zhamashi intrusion is Alaskan‐type. The mineral chemistry of the chromian spinels (Cr‐spinels) and clinopyroxenes, and the variation trend of the whole‐rock compositional plot in the (Na 2 O + K 2 O)–FeO–MgO (AFM) diagram are also supportive of this consideration. The age of the Zhamashi intrusive body, determined with sensitive high mass‐resolution ion microprobe on the zircon grains, is 513.0 ± 4.5 Ma. Parental magma of the Zhamashi intrusion is compositionally close to the primitive magma produced by partial melting of the mantle peridotite. It was differentiated by fractional crystallization at low total pressure and under H 2 O‐rich conditions in an arc environment to form all the major rock types. The concentrically zoned structure of the Zhamashi intrusive body was constructed in two stages: formation of a stratiform‐type layered sequence, followed by diapiric re‐emplacement. The occurrence of the Alaskan‐type intrusion suggests an active continental margin and Cambrian arc magmatism for the northern margin of the Qilian Block.
Heritage sites often comprise buildings and monuments constructed from rock materials. When these heritage sites require restoration, selecting replacement materials that match originals is crucial for preserving cultural authenticity. Nevertheless, sampling is generally prohibited at heritage sites, preventing the direct comparison of damaged materials with their replacement candidates. To address this issue, we established a non-destructive image analysis protocol for comparing the appearance of rock materials. Granite was selected for developing this color differentiation algorithm because it is a ubiquitous construction material and it comprises minerals with distinctive colors. Mineral proportions are, therefore, prime parameters for describing the images of granites. By appearance, granites can be sorted into the gray-white group featuring gray quartz and pink alkali feldspar and the brown-pink group characterized by pink quartz and brown alkali feldspar. In image analysis, the brown and pink pixels were distinct from the gray, white (plagioclase), and black (biotite) ones for higher R (red) values in the red, green, and blue (RGB) color model. Moreover, the dark colored pixels had a standard deviation of RGB values [δ (RGB)] that extended to higher values. Granite pixels were, therefore, first separated using a 2R/(G+B) versus δ (RGB) plot. The brown and pink pixels formed a high R array with a positive 2R/(G+B)–δ (RGB) slope, whereas the gray, white, and black pixels defined a low R array showing an inverse 2R/(G+B)–δ (RGB) correlation. The combination of δ (RGB) and (R+G+B) further separated light and dark colored pixels in a 2R/(G+B)–δ (RGB) array. At last, grayscale was applied to resolve the white and gray pixels in a low R array after isolating the black pixels. The numbers of pixels for each color were then converted to mineral proportions. More precise mineral proportions were acquired for comparison through composition analyses that required destructive procedures. The comparator values were derived by solving a matrix equation that described a bulk composition as the sum of the products of the constituent mineral compositions and their corresponding proportions. The mineral proportions from image analysis and composition analyses were within 10%. Hence, our new protocol was validated as a method for selecting proper replacement materials for restoring granite-based cultural heritage sites. Finally, the image analysis procedures were Python coded and applied to the restoration of a monument in southern Taiwan.
Cenozoic (Miocene to Pleistocene) basaltic rocks found in Shandong province of northern China include tholeiite, olivine tholeiite and alkali basalt. We present major, trace and rare earth elements data of these basalts and together with Sr-Nd isotopic data in the literatures to discuss the petrogenesis of these basalts. The basalts from Penglai area have higher K, Na and P and incompatible elements, but lower Ca, Mg and compatible elements contents than those from Changle area of Shandong province. Spidergrams indicate that Cenozoic basalts from Shandong province have geochemical characteristics similar to those of ocean island basalts (OIB) with slight positive Nb anomaly. The negative Ba, Rb and K anomalies found in the alkali basalts suggest the presence of residual phlogopite in the mantle source, indicating a metasomatic event occurred before the partial melting. The 143Nd/144Nd vs. 87Sr/86Sr plot suggested that basalts from Shandong province can be produced by MORB and EM-I components mixing. We propose that the EM-I type lithospheric mantle may have been produced by the recent H2O-CO2-fluids metasomatism and the fluids may be derived from dehydration of the subducted slab.Based on Shaw's equation, the basalts from eastern and central Shandong province have undergone different degrees of partial melting from the mantle source. Degrees of partial melting and chemical composition of basalts from Shandong province suggest that the lithosphere has thickened progressively since the Miocene. On the basis of Ar-Ar ages of this study and the fractional crystallization model proposed by Brooks and Nielsen (1982), we suggest that basalts from Changle and Penglai areas belong to different magmatic systems which have undergone fractional crystallization and evolved progressively to produce other types of basalts.
Abstract Based on new Sr-Nd-Pb isotope-compositional and lithogeochemical data combined with previously published data of the Kahnouj ophiolite, we propose a tectonic model for the Kahnouj ophiolitic complex. The Kahnouj ophiolitic complex is the largest ophiolite of the Makran zone and consists of isotropic and layered gabbros with ultramafic lenses at the bottom and sheeted dikes, and basalts and pelagic limestones on the top. A cyclic succession of isotropic and layered gabbros indicates a distinct differentiation trend of early clinopyroxene crystallization followed by hornblende. Crystal fractionation and partial melting processes are also inferred by Nd isotopic and whole-rock geochemical data (e.g., fractionation recorded by the Al 2 O 3 /TiO 2 versus Ti/1000 diagram). Geochemically, the gabbros are subalkaline and show tholeiitic features. The εNd values of + 7 to + 7.6 and initial 87 Sr/ 86 Sr ratios of 0.70352 to 0.70377 indicate a mid-ocean ridge mantle source for the gabbroic rocks. The whole rock geochemistry of the layered and isotropic gabbros suggest that they formed in a back-arc basin setting and represent MOR- to supra-subduction type ophiolites in the Neotethys Ocean during Jurassic to Early Cretaceous (156 to 121 Ma).
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