The Tarim large igneous province (TLIP), recognized as an atypical transitional LIP, contains numerous alkaline intermediate to felsic intrusions that exhibit affinities to A-type granites and are spatially and temporally associated with mafic-ultramafic intrusions hosting Fe-Ti-V oxide mineralization. The petrogenesis of these alkaline intermediate-felsic rocks and their implications for crustal growth remain, however, highly debated. An integrated approach incorporating petrography, mineral chemistry, zircon U-Pb geochronology, whole-rock major and trace element geochemistry, and Sr-Nd-Mg isotope systematics was applied to elucidate the petrogenetic history of the Xiaohaizi syenitic intrusion and its record of intracontinental crustal growth. The results show that the Xiaohaizi intrusion is composed mainly of fayalite syenite, amphibole syenite, quartz syenite, and syenite porphyry, emplaced at (283.3 ± 2.1) Ma. The whole-rock samples are characterized by high Fe-number [FeO^T/(FeO^T+MgO) = 0.76~0.98], total alkalis (Na₂O+K₂O = 8.99%~12.2%), and 10 000×Ga/Al ratios (2.83~5.10), together with low concentrations of MgO (0.05%~2.37%), MnO (0.04%~0.17%), and P₂O₅ (0.03%~0.61%). They are enriched in large-ion lithophile elements (LILEs; e.g., Rb, Ba, Th, U) and high-field-strength elements (HFSEs; e.g., Nb, Ta), and are classified as weakly peralkaline to metaluminous A-type granitoids. The late-stage syenites from Xiaohaizi yield initial (⁸⁷Sr/⁸⁶Sr)_i ratios ranging from 0.703 27 to 0.704 23 and ε_Nd(t) values from-2.70 to 3.98. These isotopic compositions overlap with those of the olivine pyroxenites from coeval mafic-ultramafic intrusions [(⁸⁷Sr/⁸⁶Sr)_i = 0.704 52~0.704 59, ε_Nd(t) =1.12 to 2.33], suggesting a common mantle plume source for both rock suites. Trace element compositions of olivine and Mg isotope data (δ²⁶Mg) further indicate that the parental magma of the Xiaohaizi syenites is an alkali basaltic melt derived from partial melting of carbonated eclogite within the mantle, which had been previously modified by silicate melts. Fractional crystallization of olivine and clinopyroxene from this alkali basaltic magma produced the olivine pyroxenites. The residual melt subsequently experienced extensive cumulate processes dominated by alkali feldspar, giving rise to the fayalite syenite, amphibole syenite, quartz syenite, and syenite porphyry. These findings demonstrate that the mantle acts not only as a thermal engine but also as the primary material source for crustal growth in intracontinental settings.