Recent second/foreign language (L2) research has witnessed the application of sociocultural tenets to L2 classrooms. This study aimed to probe whether Iranian L2 learners' engagement in ZPD-activated collaborative dialogue, or "languaging", mediates their learning process and, specifically, their appropriate use of metadiscourse to address content, organisation and audience issues in writing. English-as-a-foreign-language-writing classes at two universities were assigned to four different instructional conditions, namely, ZPD-activated collaborative, ZPD-free collaborative, fine-tuned L2-input provision and prevalent teacher-fronted approaches. The data comprised metadiscourse-oriented writing test scores, weekly writings and audio-recorded in-class collaborative dialogues of the ZPD groups. The results demonstrated the ZPD-activated collaborative writing approach significantly facilitated the learners' appropriate use of metadiscourse. Complementary interpretative analysis of the ZPD participants' metadiscourse-related episodes showed that the initially-other-regulated (undistributed) writing tasks became progressively reciprocal with less-skilled peers displaying microgenetic use of L2 metadiscourse resources and dynamically assuming more participatory roles in regulating the writing tasks. The findings suggest engagement in ZPD-activated "languaging" is crucial to shaping opportunities for L2 learners' microgenetic learning and to their longer term cognitive development. In sum, the application of sociocultural tenets and notions has shown to be highly fruitful in explaining the social genesis of L2 development as a higher order psychological functioning.