Post-eruptive processes in the Campanian Ignimbrite

Summary ¶This paper deals with the mineralogical features of the most important and widespread pyroclastic formation of Southern Italy: the Campanian Ignimbrite. This formation is characterized by four stratigraphic units, different in colour and mineralogical composition. The lowermost unit (USAF) is constituted by an incoherent, thin (few tens of cm) sequence of glass or feldspar-rich layers. The middle part of the succession is formed by two main units: a lower grey, welded unit (WGI) capped by a yellow and lithified unit (LYT) that differ in both volcanological features and mineralogical composition. Locally these units are found separately. An incoherent unit (max thickness about 2m) mainly represented by fresh glass, forms the upper part of the succession. A detailed analysis has been carried out on six sections representative of the entire succession. The WGI is feldspathized by authigenic mineralization processes, and the feldspar content ranges between 30 and 90%. The LYT has been deeply affected by zeolitization processes and the total zeolite content often reaches 60%. Based on a comparative examination of both volcanological and mineralogical data, it is hypothesized that these two units were involved in different secondary mineralization processes. The portion of the succession characterized by the highest emplacement temperatures suffered welding and feldspathization and the result of these processes gave rise to the WGI unit. In contrast, the overlying lower-temperature and incoherent portion of the deposit was affected by circulation of meteoric water, thus enhancing zeolitization and lithification. The product of this process gave rise to the LYT unit.Received July 4, 2002; revised version accepted April 23, 2003     

Discovering the stone heritage of southern Italy: technical properties of the Mondragone marble from Campania region

The results of the geological characterization of an old stone building in southern Italy, are presented here. The Mondragone marble is a slightly metamorphosed carbonate rock, which was widely employed as monumental stone in the 18th century Royal Palaces of Naples and Caserta. In this paper, for the first time, this rock has been investigated with a thorough laboratory testing program, aimed at defining its mineralogical, physical and mechanical parameters. The two most important varieties of Mondragone marble were separately tested: the yellow and grey marbles (hereafter, YM and GM, respectively). The results obtained from physical tests (open porosity, dry density, specific gravity, water absorption coefficients, ultrasonic velocity) did not show marked differences between the two materials; in contrast, the mechanical strength tests (uniaxial compressive strength, point load strength, flexural strength) showed better behaviour of YM than for GM. After a tentative comparison with other well known carbonate rocks from Italy, some considerations on the response of Mondragone marble to local weathering were conducted. In particular, the weathering typologies were related to the particular texture of the brecciated marble and the different strength resistances displayed by the various constituents of the rock.     

Piperno from Campi Flegrei: a relevant stone in the historical and monumental heritage of Naples (Italy)

This paper reports novel data concerning the volcanic formation of the Piperno, a stone that has been used for a long time in the monumental architecture of Naples and the Campanian region (Southern Italy). A careful field survey of the outcrops or underground sites so far accessible allowed us to draw a lithostratigraphic reconstruction of the formation and enabled the recognition of the three main layers exploited in past centuries and which provided dimension stones for architectural and structural purposes. Petrophysical and mineralogical characterization also enabled us to discriminate the same layers previously identified. Different scoriae/matrix ratio, scoriae dimension and welding degree are the most peculiar features of these horizons. The combination of these three parameters, even though they did not affect the use of Piperno as dimension stone, somehow determined its variable susceptibility to the weathering agents. However, a linear relationship between the exposure time and the decay of the stone itself, at a parity of weathering intensity, was not observed. This assumption was proved by verifying that the specific layers of the volcanic sequence controlled the weathering degree of Piperno from monuments disregarding their age of emplacement.     

Thermal pulse induced by emplacement of Ramba leucogranites in southern Tibet

The thermal histories of Himalayan leucogranites provide critical information for unravelling the post-collisional geodynamics of the Himalayas. The Ramba Dome is located at the intersection of the Tethyan Himalayan leucogranite belt with the Yadong–Gulu Rift and hosts several generations of granitic intrusions. Of these intrusions, the 8-Ma two-mica granites and garnet leucogranite dykes are the youngest of Himalayan leucogranites. In this study, we focus on the carbonaceous staurolite schist located ~1.3 km from the intrusion to constrain the thermal history of the aureole that marked the cessation of leucogranite magmatism. The schist contains euhedral garnet and staurolite porphyroblasts in a foliated matrix of muscovite + biotite + chlorite + plagioclase + quartz + graphite. The staurolite shows minor compositional variations from the inclusion-free core to the inclusion-rich rim. By contrast, the garnet features a distinctive bell-shaped Mn profile and increasing Mg# from the garnet core to rims. In a graphite-bearing equilibrium phase diagram for a modified bulk composition with garnet cores removed, the garnet rim composition suggests a peak temperature of ~550°C, consistent with an independent thermometer based on the Raman spectra of carbonaceous materials (RSCM; 548 ± 9°C). The P–T condition lies within the narrow low-variance field bracketed by the staurolite-in and chlorite-out boundaries, indicating minimal overstepping of staurolite nucleation and growth. On the other hand, the garnet core composition indicates 520°C at 2.5 kbar, about 40°C higher than the predicted garnet-in boundary (~480°C). This apparent temperature overstep corresponds to a small chemical affinity (<5 kJ/mol 12 O) for garnet nucleation, comparable to previous estimates. The sharp boundaries of the high-Ca sector zoning in the core indicate limited diffusion modification (~1.5 Ma if at the peak temperature). The short thermal pulse involves advective heat transfer by leucogranite emplacement, followed by rapid cooling toward the end of Himalayan magmatism and rapid exhumation likely facilitated by the Yadong–Gulu Rift.