Calcite‐Graphite Thermometry of the Franklin Marble, New Jersey Highlands

Abstract

We present new stable‐isotope data for the Mesoproterozoic Franklin Marble from outcrops along an 80‐km traverse parallel to and across strike of the structural grain of the western New Jersey Highlands. Calcite and dolomite from marble have an average δ13C of $$0.35\mbox{\textperthousand} \pm 0.73\mbox{\textperthousand} $$ PDB ($$n=46$$ ) and a more limited range than other Mesoproterozoic marbles from the Adirondacks and the Canadian Grenville Province. The small range of δ13C values from the New Jersey samples is consistent with the preservation of a primary marine isotopic signature and limited postdepositional isotopic modification, except proximal to Zn or Fe ore deposits and fault zones. Fractionations between calcite and well‐formed graphite (Δ13C[Cal‐Gr]) for analyzed Franklin Marble samples average $$3.31\mbox{\textperthousand} \pm 0.25\mbox{\textperthousand} $$ ($$n=34$$ ), and dolomite‐graphite fractionations average $$3.07\mbox{\textperthousand} \pm 0.30\mbox{\textperthousand} $$ ($$n=6$$ ). Taken together, these indicate an average temperature of $$769^{\circ }\pm 43^{\circ }$$ C during metamorphism associated with the Ottawan Orogeny in the New Jersey Highlands. Thus, carbon isotope fractionations demonstrate that the Franklin Marble was metamorphosed at granulite facies conditions. Metamorphic temperatures are relatively constant for the area sampled and overprint the metamorphosed carbonate‐hosted Zn‐Fe‐Mn ore deposits. The results of this study support recent work proposing that pressure and temperature conditions during Ottawan orogenesis did not vary greatly across faults that partition the Highlands into structural blocks.